Are there solutions to $a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$ in positive integers $(a,b,c,d)$ with $d>1$?












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Let be $a, b, c, d$ positive integers.
Consider the following equation:



$$a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$$



$(a,b,c,d)=(1,1,1,1)$ is a solution
$(a,b,c,d)=(5,4,6,1)$ is another solution.




Are there integer solutions with $d>1$?











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  • $begingroup$
    @Peter can these solutions be found only by the brute force of a computer?
    $endgroup$
    – user631773
    Jan 5 at 18:08










  • $begingroup$
    $(2, 2, 2, 2)$ is also a solution where $d>1$.
    $endgroup$
    – EuxhenH
    Jan 5 at 18:23










  • $begingroup$
    (5,4,6,1) is not a solution
    $endgroup$
    – Will Jagy
    Jan 5 at 18:33
















0












$begingroup$


Let be $a, b, c, d$ positive integers.
Consider the following equation:



$$a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$$



$(a,b,c,d)=(1,1,1,1)$ is a solution
$(a,b,c,d)=(5,4,6,1)$ is another solution.




Are there integer solutions with $d>1$?











share|cite|improve this question











$endgroup$












  • $begingroup$
    @Peter can these solutions be found only by the brute force of a computer?
    $endgroup$
    – user631773
    Jan 5 at 18:08










  • $begingroup$
    $(2, 2, 2, 2)$ is also a solution where $d>1$.
    $endgroup$
    – EuxhenH
    Jan 5 at 18:23










  • $begingroup$
    (5,4,6,1) is not a solution
    $endgroup$
    – Will Jagy
    Jan 5 at 18:33














0












0








0


1



$begingroup$


Let be $a, b, c, d$ positive integers.
Consider the following equation:



$$a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$$



$(a,b,c,d)=(1,1,1,1)$ is a solution
$(a,b,c,d)=(5,4,6,1)$ is another solution.




Are there integer solutions with $d>1$?











share|cite|improve this question











$endgroup$




Let be $a, b, c, d$ positive integers.
Consider the following equation:



$$a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$$



$(a,b,c,d)=(1,1,1,1)$ is a solution
$(a,b,c,d)=(5,4,6,1)$ is another solution.




Are there integer solutions with $d>1$?








number-theory






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edited Jan 5 at 18:21









Blue

49.1k870156




49.1k870156










asked Jan 5 at 18:03









user631773user631773

11




11












  • $begingroup$
    @Peter can these solutions be found only by the brute force of a computer?
    $endgroup$
    – user631773
    Jan 5 at 18:08










  • $begingroup$
    $(2, 2, 2, 2)$ is also a solution where $d>1$.
    $endgroup$
    – EuxhenH
    Jan 5 at 18:23










  • $begingroup$
    (5,4,6,1) is not a solution
    $endgroup$
    – Will Jagy
    Jan 5 at 18:33


















  • $begingroup$
    @Peter can these solutions be found only by the brute force of a computer?
    $endgroup$
    – user631773
    Jan 5 at 18:08










  • $begingroup$
    $(2, 2, 2, 2)$ is also a solution where $d>1$.
    $endgroup$
    – EuxhenH
    Jan 5 at 18:23










  • $begingroup$
    (5,4,6,1) is not a solution
    $endgroup$
    – Will Jagy
    Jan 5 at 18:33
















$begingroup$
@Peter can these solutions be found only by the brute force of a computer?
$endgroup$
– user631773
Jan 5 at 18:08




$begingroup$
@Peter can these solutions be found only by the brute force of a computer?
$endgroup$
– user631773
Jan 5 at 18:08












$begingroup$
$(2, 2, 2, 2)$ is also a solution where $d>1$.
$endgroup$
– EuxhenH
Jan 5 at 18:23




$begingroup$
$(2, 2, 2, 2)$ is also a solution where $d>1$.
$endgroup$
– EuxhenH
Jan 5 at 18:23












$begingroup$
(5,4,6,1) is not a solution
$endgroup$
– Will Jagy
Jan 5 at 18:33




$begingroup$
(5,4,6,1) is not a solution
$endgroup$
– Will Jagy
Jan 5 at 18:33










4 Answers
4






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1












$begingroup$

Let $b=c=d=t$. The expression simplifies to $a=t$. Hence, you can get many solutions where $d>1$.






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    1












    $begingroup$

    It seems that $frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$ is not equal to 5 when $b=4$, $c=6$ and $d=1$, no ?



    Brute-forcing can help you to get some solutions :



    (1,1,1,1)
    (2,2,2,2)
    (3,2,2,4)
    (3,3,3,3)
    (5,3,5,6)
    (8,3,9,6)
    (4,4,4,4)
    (6,4,4,8)
    (5,5,3,6)
    (5,5,5,5)
    (6,6,6,6)
    (7,7,7,7)
    (8,7,9,4)
    (8,8,8,8)
    (8,9,3,6)
    (8,9,7,4)
    (9,9,9,9)





    share|cite|improve this answer











    $endgroup$





















      0












      $begingroup$

      I get it now. Begin with any triple such that $gcd(b,c,d) = 1.$ The fraction is not necessarily an integer. However, there is a smallest multiplier, call it $k,$ such that using $(kb,kc,kd)$ does give integer $a,$ also $gcd(a, kb,kc,kd) = 1.$ Also, the algorithm to find $k$ is easy.
      $$ k = frac{2b^2+2c^2+d^2}{gcd left( 2b^2+2c^2+d^2 ;, ;2b^3+2c^3+d^3 right)} $$



      In particular, the mistaken triple (6,4,1) must be multiplied by 35 to get $a$ integral.



      original     6   4   1 mult    35 gives      187       210     140      35


      =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=



      jagy@phobeusjunior:~$ ./mse 
      original 1 1 1 mult 1 gives 1 1 1 1
      original 1 1 2 mult 2 gives 3 2 2 4
      original 1 1 3 mult 13 gives 31 13 13 39
      original 1 1 4 mult 5 gives 17 5 5 20
      original 1 1 5 mult 29 gives 129 29 29 145
      original 1 1 6 mult 2 gives 11 2 2 12
      original 2 1 1 mult 11 gives 19 22 11 11
      original 2 1 2 mult 7 gives 13 14 7 14
      original 2 1 3 mult 19 gives 45 38 19 57
      original 2 1 4 mult 13 gives 41 26 13 52
      original 2 1 5 mult 35 gives 143 70 35 175
      original 2 1 6 mult 23 gives 117 46 23 138
      original 2 2 1 mult 17 gives 33 34 34 17
      original 2 2 3 mult 25 gives 59 50 50 75
      original 2 2 5 mult 41 gives 157 82 82 205
      original 3 1 1 mult 7 gives 19 21 7 7
      original 3 1 2 mult 3 gives 8 9 3 6
      original 3 1 3 mult 29 gives 83 87 29 87
      original 3 1 4 mult 3 gives 10 9 3 12
      original 3 1 5 mult 45 gives 181 135 45 225
      original 3 1 6 mult 7 gives 34 21 7 42
      original 3 2 1 mult 27 gives 71 81 54 27
      original 3 2 2 mult 5 gives 13 15 10 10
      original 3 2 3 mult 35 gives 97 105 70 105
      original 3 2 4 mult 21 gives 67 63 42 84
      original 3 2 5 mult 17 gives 65 51 34 85
      original 3 2 6 mult 31 gives 143 93 62 186
      original 3 3 1 mult 37 gives 109 111 111 37
      original 3 3 2 mult 10 gives 29 30 30 20
      original 3 3 4 mult 13 gives 43 39 39 52
      original 3 3 5 mult 61 gives 233 183 183 305
      original 4 1 1 mult 35 gives 131 140 35 35
      original 4 1 2 mult 19 gives 69 76 19 38
      original 4 1 3 mult 43 gives 157 172 43 129
      original 4 1 4 mult 25 gives 97 100 25 100
      original 4 1 5 mult 59 gives 255 236 59 295
      original 4 1 6 mult 35 gives 173 140 35 210
      original 4 2 1 mult 41 gives 145 164 82 41
      original 4 2 3 mult 49 gives 171 196 98 147
      original 4 2 5 mult 65 gives 269 260 130 325
      original 4 3 1 mult 17 gives 61 68 51 17
      original 4 3 2 mult 27 gives 95 108 81 54
      original 4 3 3 mult 59 gives 209 236 177 177
      original 4 3 4 mult 11 gives 41 44 33 44
      original 4 3 5 mult 75 gives 307 300 225 375
      original 4 3 6 mult 43 gives 199 172 129 258
      original 4 4 1 mult 65 gives 257 260 260 65
      original 4 4 3 mult 73 gives 283 292 292 219
      original 4 4 5 mult 89 gives 381 356 356 445
      original 5 1 1 mult 53 gives 253 265 53 53
      original 5 1 2 mult 14 gives 65 70 14 28
      original 5 1 3 mult 61 gives 279 305 61 183
      original 5 1 4 mult 17 gives 79 85 17 68
      original 5 1 5 mult 77 gives 377 385 77 385
      original 5 1 6 mult 22 gives 117 110 22 132
      original 5 2 1 mult 59 gives 267 295 118 59
      original 5 2 2 mult 31 gives 137 155 62 62
      original 5 2 3 mult 67 gives 293 335 134 201
      original 5 2 4 mult 37 gives 165 185 74 148
      original 5 2 5 mult 83 gives 391 415 166 415
      original 5 2 6 mult 47 gives 241 235 94 282
      original 5 3 1 mult 69 gives 305 345 207 69
      original 5 3 2 mult 3 gives 13 15 9 6
      original 5 3 3 mult 77 gives 331 385 231 231
      original 5 3 4 mult 21 gives 92 105 63 84
      original 5 3 5 mult 31 gives 143 155 93 155
      original 5 3 6 mult 1 gives 5 5 3 6
      original 5 4 1 mult 83 gives 379 415 332 83
      original 5 4 2 mult 43 gives 193 215 172 86
      original 5 4 3 mult 91 gives 405 455 364 273
      original 5 4 4 mult 49 gives 221 245 196 196
      original 5 4 5 mult 107 gives 503 535 428 535
      original 5 4 6 mult 59 gives 297 295 236 354
      original 5 5 1 mult 101 gives 501 505 505 101
      original 5 5 2 mult 26 gives 127 130 130 52
      original 5 5 3 mult 109 gives 527 545 545 327
      original 5 5 4 mult 29 gives 141 145 145 116
      original 5 5 6 mult 34 gives 179 170 170 204
      original 6 1 1 mult 5 gives 29 30 5 5
      original 6 1 2 mult 3 gives 17 18 3 6
      original 6 1 3 mult 83 gives 461 498 83 249
      original 6 1 4 mult 15 gives 83 90 15 60
      original 6 1 5 mult 99 gives 559 594 99 495
      original 6 1 6 mult 11 gives 65 66 11 66
      original 6 2 1 mult 81 gives 449 486 162 81
      original 6 2 3 mult 89 gives 475 534 178 267
      original 6 2 5 mult 35 gives 191 210 70 175
      original 6 3 1 mult 91 gives 487 546 273 91
      original 6 3 2 mult 47 gives 247 282 141 94
      original 6 3 4 mult 53 gives 275 318 159 212
      original 6 3 5 mult 115 gives 611 690 345 575
      original 6 4 1 mult 35 gives 187 210 140 35
      original 6 4 3 mult 113 gives 587 678 452 339
      original 6 4 5 mult 129 gives 685 774 516 645
      original 6 5 1 mult 123 gives 683 738 615 123
      original 6 5 2 mult 21 gives 115 126 105 42
      original 6 5 3 mult 131 gives 709 786 655 393
      original 6 5 4 mult 69 gives 373 414 345 276
      original 6 5 5 mult 49 gives 269 294 245 245
      original 6 5 6 mult 79 gives 449 474 395 474
      original 6 6 1 mult 29 gives 173 174 174 29
      original 6 6 5 mult 169 gives 989 1014 1014 845





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        0












        $begingroup$

        For the equation.



        $$a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$$



        You can record this parameterization.



        $$a=p^3-3s^3$$



        $$b=p^3+sp^2-9s^3$$



        $$c=p(p^2-6s^2)$$



        $$d=p^3-2sp^2+9s^3$$






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          4 Answers
          4






          active

          oldest

          votes








          4 Answers
          4






          active

          oldest

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          active

          oldest

          votes






          active

          oldest

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          1












          $begingroup$

          Let $b=c=d=t$. The expression simplifies to $a=t$. Hence, you can get many solutions where $d>1$.






          share|cite|improve this answer









          $endgroup$


















            1












            $begingroup$

            Let $b=c=d=t$. The expression simplifies to $a=t$. Hence, you can get many solutions where $d>1$.






            share|cite|improve this answer









            $endgroup$
















              1












              1








              1





              $begingroup$

              Let $b=c=d=t$. The expression simplifies to $a=t$. Hence, you can get many solutions where $d>1$.






              share|cite|improve this answer









              $endgroup$



              Let $b=c=d=t$. The expression simplifies to $a=t$. Hence, you can get many solutions where $d>1$.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Jan 5 at 18:25









              EuxhenHEuxhenH

              482210




              482210























                  1












                  $begingroup$

                  It seems that $frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$ is not equal to 5 when $b=4$, $c=6$ and $d=1$, no ?



                  Brute-forcing can help you to get some solutions :



                  (1,1,1,1)
                  (2,2,2,2)
                  (3,2,2,4)
                  (3,3,3,3)
                  (5,3,5,6)
                  (8,3,9,6)
                  (4,4,4,4)
                  (6,4,4,8)
                  (5,5,3,6)
                  (5,5,5,5)
                  (6,6,6,6)
                  (7,7,7,7)
                  (8,7,9,4)
                  (8,8,8,8)
                  (8,9,3,6)
                  (8,9,7,4)
                  (9,9,9,9)





                  share|cite|improve this answer











                  $endgroup$


















                    1












                    $begingroup$

                    It seems that $frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$ is not equal to 5 when $b=4$, $c=6$ and $d=1$, no ?



                    Brute-forcing can help you to get some solutions :



                    (1,1,1,1)
                    (2,2,2,2)
                    (3,2,2,4)
                    (3,3,3,3)
                    (5,3,5,6)
                    (8,3,9,6)
                    (4,4,4,4)
                    (6,4,4,8)
                    (5,5,3,6)
                    (5,5,5,5)
                    (6,6,6,6)
                    (7,7,7,7)
                    (8,7,9,4)
                    (8,8,8,8)
                    (8,9,3,6)
                    (8,9,7,4)
                    (9,9,9,9)





                    share|cite|improve this answer











                    $endgroup$
















                      1












                      1








                      1





                      $begingroup$

                      It seems that $frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$ is not equal to 5 when $b=4$, $c=6$ and $d=1$, no ?



                      Brute-forcing can help you to get some solutions :



                      (1,1,1,1)
                      (2,2,2,2)
                      (3,2,2,4)
                      (3,3,3,3)
                      (5,3,5,6)
                      (8,3,9,6)
                      (4,4,4,4)
                      (6,4,4,8)
                      (5,5,3,6)
                      (5,5,5,5)
                      (6,6,6,6)
                      (7,7,7,7)
                      (8,7,9,4)
                      (8,8,8,8)
                      (8,9,3,6)
                      (8,9,7,4)
                      (9,9,9,9)





                      share|cite|improve this answer











                      $endgroup$



                      It seems that $frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$ is not equal to 5 when $b=4$, $c=6$ and $d=1$, no ?



                      Brute-forcing can help you to get some solutions :



                      (1,1,1,1)
                      (2,2,2,2)
                      (3,2,2,4)
                      (3,3,3,3)
                      (5,3,5,6)
                      (8,3,9,6)
                      (4,4,4,4)
                      (6,4,4,8)
                      (5,5,3,6)
                      (5,5,5,5)
                      (6,6,6,6)
                      (7,7,7,7)
                      (8,7,9,4)
                      (8,8,8,8)
                      (8,9,3,6)
                      (8,9,7,4)
                      (9,9,9,9)






                      share|cite|improve this answer














                      share|cite|improve this answer



                      share|cite|improve this answer








                      edited Jan 5 at 18:31

























                      answered Jan 5 at 18:22









                      YoshiYoshi

                      133




                      133























                          0












                          $begingroup$

                          I get it now. Begin with any triple such that $gcd(b,c,d) = 1.$ The fraction is not necessarily an integer. However, there is a smallest multiplier, call it $k,$ such that using $(kb,kc,kd)$ does give integer $a,$ also $gcd(a, kb,kc,kd) = 1.$ Also, the algorithm to find $k$ is easy.
                          $$ k = frac{2b^2+2c^2+d^2}{gcd left( 2b^2+2c^2+d^2 ;, ;2b^3+2c^3+d^3 right)} $$



                          In particular, the mistaken triple (6,4,1) must be multiplied by 35 to get $a$ integral.



                          original     6   4   1 mult    35 gives      187       210     140      35


                          =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=



                          jagy@phobeusjunior:~$ ./mse 
                          original 1 1 1 mult 1 gives 1 1 1 1
                          original 1 1 2 mult 2 gives 3 2 2 4
                          original 1 1 3 mult 13 gives 31 13 13 39
                          original 1 1 4 mult 5 gives 17 5 5 20
                          original 1 1 5 mult 29 gives 129 29 29 145
                          original 1 1 6 mult 2 gives 11 2 2 12
                          original 2 1 1 mult 11 gives 19 22 11 11
                          original 2 1 2 mult 7 gives 13 14 7 14
                          original 2 1 3 mult 19 gives 45 38 19 57
                          original 2 1 4 mult 13 gives 41 26 13 52
                          original 2 1 5 mult 35 gives 143 70 35 175
                          original 2 1 6 mult 23 gives 117 46 23 138
                          original 2 2 1 mult 17 gives 33 34 34 17
                          original 2 2 3 mult 25 gives 59 50 50 75
                          original 2 2 5 mult 41 gives 157 82 82 205
                          original 3 1 1 mult 7 gives 19 21 7 7
                          original 3 1 2 mult 3 gives 8 9 3 6
                          original 3 1 3 mult 29 gives 83 87 29 87
                          original 3 1 4 mult 3 gives 10 9 3 12
                          original 3 1 5 mult 45 gives 181 135 45 225
                          original 3 1 6 mult 7 gives 34 21 7 42
                          original 3 2 1 mult 27 gives 71 81 54 27
                          original 3 2 2 mult 5 gives 13 15 10 10
                          original 3 2 3 mult 35 gives 97 105 70 105
                          original 3 2 4 mult 21 gives 67 63 42 84
                          original 3 2 5 mult 17 gives 65 51 34 85
                          original 3 2 6 mult 31 gives 143 93 62 186
                          original 3 3 1 mult 37 gives 109 111 111 37
                          original 3 3 2 mult 10 gives 29 30 30 20
                          original 3 3 4 mult 13 gives 43 39 39 52
                          original 3 3 5 mult 61 gives 233 183 183 305
                          original 4 1 1 mult 35 gives 131 140 35 35
                          original 4 1 2 mult 19 gives 69 76 19 38
                          original 4 1 3 mult 43 gives 157 172 43 129
                          original 4 1 4 mult 25 gives 97 100 25 100
                          original 4 1 5 mult 59 gives 255 236 59 295
                          original 4 1 6 mult 35 gives 173 140 35 210
                          original 4 2 1 mult 41 gives 145 164 82 41
                          original 4 2 3 mult 49 gives 171 196 98 147
                          original 4 2 5 mult 65 gives 269 260 130 325
                          original 4 3 1 mult 17 gives 61 68 51 17
                          original 4 3 2 mult 27 gives 95 108 81 54
                          original 4 3 3 mult 59 gives 209 236 177 177
                          original 4 3 4 mult 11 gives 41 44 33 44
                          original 4 3 5 mult 75 gives 307 300 225 375
                          original 4 3 6 mult 43 gives 199 172 129 258
                          original 4 4 1 mult 65 gives 257 260 260 65
                          original 4 4 3 mult 73 gives 283 292 292 219
                          original 4 4 5 mult 89 gives 381 356 356 445
                          original 5 1 1 mult 53 gives 253 265 53 53
                          original 5 1 2 mult 14 gives 65 70 14 28
                          original 5 1 3 mult 61 gives 279 305 61 183
                          original 5 1 4 mult 17 gives 79 85 17 68
                          original 5 1 5 mult 77 gives 377 385 77 385
                          original 5 1 6 mult 22 gives 117 110 22 132
                          original 5 2 1 mult 59 gives 267 295 118 59
                          original 5 2 2 mult 31 gives 137 155 62 62
                          original 5 2 3 mult 67 gives 293 335 134 201
                          original 5 2 4 mult 37 gives 165 185 74 148
                          original 5 2 5 mult 83 gives 391 415 166 415
                          original 5 2 6 mult 47 gives 241 235 94 282
                          original 5 3 1 mult 69 gives 305 345 207 69
                          original 5 3 2 mult 3 gives 13 15 9 6
                          original 5 3 3 mult 77 gives 331 385 231 231
                          original 5 3 4 mult 21 gives 92 105 63 84
                          original 5 3 5 mult 31 gives 143 155 93 155
                          original 5 3 6 mult 1 gives 5 5 3 6
                          original 5 4 1 mult 83 gives 379 415 332 83
                          original 5 4 2 mult 43 gives 193 215 172 86
                          original 5 4 3 mult 91 gives 405 455 364 273
                          original 5 4 4 mult 49 gives 221 245 196 196
                          original 5 4 5 mult 107 gives 503 535 428 535
                          original 5 4 6 mult 59 gives 297 295 236 354
                          original 5 5 1 mult 101 gives 501 505 505 101
                          original 5 5 2 mult 26 gives 127 130 130 52
                          original 5 5 3 mult 109 gives 527 545 545 327
                          original 5 5 4 mult 29 gives 141 145 145 116
                          original 5 5 6 mult 34 gives 179 170 170 204
                          original 6 1 1 mult 5 gives 29 30 5 5
                          original 6 1 2 mult 3 gives 17 18 3 6
                          original 6 1 3 mult 83 gives 461 498 83 249
                          original 6 1 4 mult 15 gives 83 90 15 60
                          original 6 1 5 mult 99 gives 559 594 99 495
                          original 6 1 6 mult 11 gives 65 66 11 66
                          original 6 2 1 mult 81 gives 449 486 162 81
                          original 6 2 3 mult 89 gives 475 534 178 267
                          original 6 2 5 mult 35 gives 191 210 70 175
                          original 6 3 1 mult 91 gives 487 546 273 91
                          original 6 3 2 mult 47 gives 247 282 141 94
                          original 6 3 4 mult 53 gives 275 318 159 212
                          original 6 3 5 mult 115 gives 611 690 345 575
                          original 6 4 1 mult 35 gives 187 210 140 35
                          original 6 4 3 mult 113 gives 587 678 452 339
                          original 6 4 5 mult 129 gives 685 774 516 645
                          original 6 5 1 mult 123 gives 683 738 615 123
                          original 6 5 2 mult 21 gives 115 126 105 42
                          original 6 5 3 mult 131 gives 709 786 655 393
                          original 6 5 4 mult 69 gives 373 414 345 276
                          original 6 5 5 mult 49 gives 269 294 245 245
                          original 6 5 6 mult 79 gives 449 474 395 474
                          original 6 6 1 mult 29 gives 173 174 174 29
                          original 6 6 5 mult 169 gives 989 1014 1014 845





                          share|cite|improve this answer











                          $endgroup$


















                            0












                            $begingroup$

                            I get it now. Begin with any triple such that $gcd(b,c,d) = 1.$ The fraction is not necessarily an integer. However, there is a smallest multiplier, call it $k,$ such that using $(kb,kc,kd)$ does give integer $a,$ also $gcd(a, kb,kc,kd) = 1.$ Also, the algorithm to find $k$ is easy.
                            $$ k = frac{2b^2+2c^2+d^2}{gcd left( 2b^2+2c^2+d^2 ;, ;2b^3+2c^3+d^3 right)} $$



                            In particular, the mistaken triple (6,4,1) must be multiplied by 35 to get $a$ integral.



                            original     6   4   1 mult    35 gives      187       210     140      35


                            =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=



                            jagy@phobeusjunior:~$ ./mse 
                            original 1 1 1 mult 1 gives 1 1 1 1
                            original 1 1 2 mult 2 gives 3 2 2 4
                            original 1 1 3 mult 13 gives 31 13 13 39
                            original 1 1 4 mult 5 gives 17 5 5 20
                            original 1 1 5 mult 29 gives 129 29 29 145
                            original 1 1 6 mult 2 gives 11 2 2 12
                            original 2 1 1 mult 11 gives 19 22 11 11
                            original 2 1 2 mult 7 gives 13 14 7 14
                            original 2 1 3 mult 19 gives 45 38 19 57
                            original 2 1 4 mult 13 gives 41 26 13 52
                            original 2 1 5 mult 35 gives 143 70 35 175
                            original 2 1 6 mult 23 gives 117 46 23 138
                            original 2 2 1 mult 17 gives 33 34 34 17
                            original 2 2 3 mult 25 gives 59 50 50 75
                            original 2 2 5 mult 41 gives 157 82 82 205
                            original 3 1 1 mult 7 gives 19 21 7 7
                            original 3 1 2 mult 3 gives 8 9 3 6
                            original 3 1 3 mult 29 gives 83 87 29 87
                            original 3 1 4 mult 3 gives 10 9 3 12
                            original 3 1 5 mult 45 gives 181 135 45 225
                            original 3 1 6 mult 7 gives 34 21 7 42
                            original 3 2 1 mult 27 gives 71 81 54 27
                            original 3 2 2 mult 5 gives 13 15 10 10
                            original 3 2 3 mult 35 gives 97 105 70 105
                            original 3 2 4 mult 21 gives 67 63 42 84
                            original 3 2 5 mult 17 gives 65 51 34 85
                            original 3 2 6 mult 31 gives 143 93 62 186
                            original 3 3 1 mult 37 gives 109 111 111 37
                            original 3 3 2 mult 10 gives 29 30 30 20
                            original 3 3 4 mult 13 gives 43 39 39 52
                            original 3 3 5 mult 61 gives 233 183 183 305
                            original 4 1 1 mult 35 gives 131 140 35 35
                            original 4 1 2 mult 19 gives 69 76 19 38
                            original 4 1 3 mult 43 gives 157 172 43 129
                            original 4 1 4 mult 25 gives 97 100 25 100
                            original 4 1 5 mult 59 gives 255 236 59 295
                            original 4 1 6 mult 35 gives 173 140 35 210
                            original 4 2 1 mult 41 gives 145 164 82 41
                            original 4 2 3 mult 49 gives 171 196 98 147
                            original 4 2 5 mult 65 gives 269 260 130 325
                            original 4 3 1 mult 17 gives 61 68 51 17
                            original 4 3 2 mult 27 gives 95 108 81 54
                            original 4 3 3 mult 59 gives 209 236 177 177
                            original 4 3 4 mult 11 gives 41 44 33 44
                            original 4 3 5 mult 75 gives 307 300 225 375
                            original 4 3 6 mult 43 gives 199 172 129 258
                            original 4 4 1 mult 65 gives 257 260 260 65
                            original 4 4 3 mult 73 gives 283 292 292 219
                            original 4 4 5 mult 89 gives 381 356 356 445
                            original 5 1 1 mult 53 gives 253 265 53 53
                            original 5 1 2 mult 14 gives 65 70 14 28
                            original 5 1 3 mult 61 gives 279 305 61 183
                            original 5 1 4 mult 17 gives 79 85 17 68
                            original 5 1 5 mult 77 gives 377 385 77 385
                            original 5 1 6 mult 22 gives 117 110 22 132
                            original 5 2 1 mult 59 gives 267 295 118 59
                            original 5 2 2 mult 31 gives 137 155 62 62
                            original 5 2 3 mult 67 gives 293 335 134 201
                            original 5 2 4 mult 37 gives 165 185 74 148
                            original 5 2 5 mult 83 gives 391 415 166 415
                            original 5 2 6 mult 47 gives 241 235 94 282
                            original 5 3 1 mult 69 gives 305 345 207 69
                            original 5 3 2 mult 3 gives 13 15 9 6
                            original 5 3 3 mult 77 gives 331 385 231 231
                            original 5 3 4 mult 21 gives 92 105 63 84
                            original 5 3 5 mult 31 gives 143 155 93 155
                            original 5 3 6 mult 1 gives 5 5 3 6
                            original 5 4 1 mult 83 gives 379 415 332 83
                            original 5 4 2 mult 43 gives 193 215 172 86
                            original 5 4 3 mult 91 gives 405 455 364 273
                            original 5 4 4 mult 49 gives 221 245 196 196
                            original 5 4 5 mult 107 gives 503 535 428 535
                            original 5 4 6 mult 59 gives 297 295 236 354
                            original 5 5 1 mult 101 gives 501 505 505 101
                            original 5 5 2 mult 26 gives 127 130 130 52
                            original 5 5 3 mult 109 gives 527 545 545 327
                            original 5 5 4 mult 29 gives 141 145 145 116
                            original 5 5 6 mult 34 gives 179 170 170 204
                            original 6 1 1 mult 5 gives 29 30 5 5
                            original 6 1 2 mult 3 gives 17 18 3 6
                            original 6 1 3 mult 83 gives 461 498 83 249
                            original 6 1 4 mult 15 gives 83 90 15 60
                            original 6 1 5 mult 99 gives 559 594 99 495
                            original 6 1 6 mult 11 gives 65 66 11 66
                            original 6 2 1 mult 81 gives 449 486 162 81
                            original 6 2 3 mult 89 gives 475 534 178 267
                            original 6 2 5 mult 35 gives 191 210 70 175
                            original 6 3 1 mult 91 gives 487 546 273 91
                            original 6 3 2 mult 47 gives 247 282 141 94
                            original 6 3 4 mult 53 gives 275 318 159 212
                            original 6 3 5 mult 115 gives 611 690 345 575
                            original 6 4 1 mult 35 gives 187 210 140 35
                            original 6 4 3 mult 113 gives 587 678 452 339
                            original 6 4 5 mult 129 gives 685 774 516 645
                            original 6 5 1 mult 123 gives 683 738 615 123
                            original 6 5 2 mult 21 gives 115 126 105 42
                            original 6 5 3 mult 131 gives 709 786 655 393
                            original 6 5 4 mult 69 gives 373 414 345 276
                            original 6 5 5 mult 49 gives 269 294 245 245
                            original 6 5 6 mult 79 gives 449 474 395 474
                            original 6 6 1 mult 29 gives 173 174 174 29
                            original 6 6 5 mult 169 gives 989 1014 1014 845





                            share|cite|improve this answer











                            $endgroup$
















                              0












                              0








                              0





                              $begingroup$

                              I get it now. Begin with any triple such that $gcd(b,c,d) = 1.$ The fraction is not necessarily an integer. However, there is a smallest multiplier, call it $k,$ such that using $(kb,kc,kd)$ does give integer $a,$ also $gcd(a, kb,kc,kd) = 1.$ Also, the algorithm to find $k$ is easy.
                              $$ k = frac{2b^2+2c^2+d^2}{gcd left( 2b^2+2c^2+d^2 ;, ;2b^3+2c^3+d^3 right)} $$



                              In particular, the mistaken triple (6,4,1) must be multiplied by 35 to get $a$ integral.



                              original     6   4   1 mult    35 gives      187       210     140      35


                              =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=



                              jagy@phobeusjunior:~$ ./mse 
                              original 1 1 1 mult 1 gives 1 1 1 1
                              original 1 1 2 mult 2 gives 3 2 2 4
                              original 1 1 3 mult 13 gives 31 13 13 39
                              original 1 1 4 mult 5 gives 17 5 5 20
                              original 1 1 5 mult 29 gives 129 29 29 145
                              original 1 1 6 mult 2 gives 11 2 2 12
                              original 2 1 1 mult 11 gives 19 22 11 11
                              original 2 1 2 mult 7 gives 13 14 7 14
                              original 2 1 3 mult 19 gives 45 38 19 57
                              original 2 1 4 mult 13 gives 41 26 13 52
                              original 2 1 5 mult 35 gives 143 70 35 175
                              original 2 1 6 mult 23 gives 117 46 23 138
                              original 2 2 1 mult 17 gives 33 34 34 17
                              original 2 2 3 mult 25 gives 59 50 50 75
                              original 2 2 5 mult 41 gives 157 82 82 205
                              original 3 1 1 mult 7 gives 19 21 7 7
                              original 3 1 2 mult 3 gives 8 9 3 6
                              original 3 1 3 mult 29 gives 83 87 29 87
                              original 3 1 4 mult 3 gives 10 9 3 12
                              original 3 1 5 mult 45 gives 181 135 45 225
                              original 3 1 6 mult 7 gives 34 21 7 42
                              original 3 2 1 mult 27 gives 71 81 54 27
                              original 3 2 2 mult 5 gives 13 15 10 10
                              original 3 2 3 mult 35 gives 97 105 70 105
                              original 3 2 4 mult 21 gives 67 63 42 84
                              original 3 2 5 mult 17 gives 65 51 34 85
                              original 3 2 6 mult 31 gives 143 93 62 186
                              original 3 3 1 mult 37 gives 109 111 111 37
                              original 3 3 2 mult 10 gives 29 30 30 20
                              original 3 3 4 mult 13 gives 43 39 39 52
                              original 3 3 5 mult 61 gives 233 183 183 305
                              original 4 1 1 mult 35 gives 131 140 35 35
                              original 4 1 2 mult 19 gives 69 76 19 38
                              original 4 1 3 mult 43 gives 157 172 43 129
                              original 4 1 4 mult 25 gives 97 100 25 100
                              original 4 1 5 mult 59 gives 255 236 59 295
                              original 4 1 6 mult 35 gives 173 140 35 210
                              original 4 2 1 mult 41 gives 145 164 82 41
                              original 4 2 3 mult 49 gives 171 196 98 147
                              original 4 2 5 mult 65 gives 269 260 130 325
                              original 4 3 1 mult 17 gives 61 68 51 17
                              original 4 3 2 mult 27 gives 95 108 81 54
                              original 4 3 3 mult 59 gives 209 236 177 177
                              original 4 3 4 mult 11 gives 41 44 33 44
                              original 4 3 5 mult 75 gives 307 300 225 375
                              original 4 3 6 mult 43 gives 199 172 129 258
                              original 4 4 1 mult 65 gives 257 260 260 65
                              original 4 4 3 mult 73 gives 283 292 292 219
                              original 4 4 5 mult 89 gives 381 356 356 445
                              original 5 1 1 mult 53 gives 253 265 53 53
                              original 5 1 2 mult 14 gives 65 70 14 28
                              original 5 1 3 mult 61 gives 279 305 61 183
                              original 5 1 4 mult 17 gives 79 85 17 68
                              original 5 1 5 mult 77 gives 377 385 77 385
                              original 5 1 6 mult 22 gives 117 110 22 132
                              original 5 2 1 mult 59 gives 267 295 118 59
                              original 5 2 2 mult 31 gives 137 155 62 62
                              original 5 2 3 mult 67 gives 293 335 134 201
                              original 5 2 4 mult 37 gives 165 185 74 148
                              original 5 2 5 mult 83 gives 391 415 166 415
                              original 5 2 6 mult 47 gives 241 235 94 282
                              original 5 3 1 mult 69 gives 305 345 207 69
                              original 5 3 2 mult 3 gives 13 15 9 6
                              original 5 3 3 mult 77 gives 331 385 231 231
                              original 5 3 4 mult 21 gives 92 105 63 84
                              original 5 3 5 mult 31 gives 143 155 93 155
                              original 5 3 6 mult 1 gives 5 5 3 6
                              original 5 4 1 mult 83 gives 379 415 332 83
                              original 5 4 2 mult 43 gives 193 215 172 86
                              original 5 4 3 mult 91 gives 405 455 364 273
                              original 5 4 4 mult 49 gives 221 245 196 196
                              original 5 4 5 mult 107 gives 503 535 428 535
                              original 5 4 6 mult 59 gives 297 295 236 354
                              original 5 5 1 mult 101 gives 501 505 505 101
                              original 5 5 2 mult 26 gives 127 130 130 52
                              original 5 5 3 mult 109 gives 527 545 545 327
                              original 5 5 4 mult 29 gives 141 145 145 116
                              original 5 5 6 mult 34 gives 179 170 170 204
                              original 6 1 1 mult 5 gives 29 30 5 5
                              original 6 1 2 mult 3 gives 17 18 3 6
                              original 6 1 3 mult 83 gives 461 498 83 249
                              original 6 1 4 mult 15 gives 83 90 15 60
                              original 6 1 5 mult 99 gives 559 594 99 495
                              original 6 1 6 mult 11 gives 65 66 11 66
                              original 6 2 1 mult 81 gives 449 486 162 81
                              original 6 2 3 mult 89 gives 475 534 178 267
                              original 6 2 5 mult 35 gives 191 210 70 175
                              original 6 3 1 mult 91 gives 487 546 273 91
                              original 6 3 2 mult 47 gives 247 282 141 94
                              original 6 3 4 mult 53 gives 275 318 159 212
                              original 6 3 5 mult 115 gives 611 690 345 575
                              original 6 4 1 mult 35 gives 187 210 140 35
                              original 6 4 3 mult 113 gives 587 678 452 339
                              original 6 4 5 mult 129 gives 685 774 516 645
                              original 6 5 1 mult 123 gives 683 738 615 123
                              original 6 5 2 mult 21 gives 115 126 105 42
                              original 6 5 3 mult 131 gives 709 786 655 393
                              original 6 5 4 mult 69 gives 373 414 345 276
                              original 6 5 5 mult 49 gives 269 294 245 245
                              original 6 5 6 mult 79 gives 449 474 395 474
                              original 6 6 1 mult 29 gives 173 174 174 29
                              original 6 6 5 mult 169 gives 989 1014 1014 845





                              share|cite|improve this answer











                              $endgroup$



                              I get it now. Begin with any triple such that $gcd(b,c,d) = 1.$ The fraction is not necessarily an integer. However, there is a smallest multiplier, call it $k,$ such that using $(kb,kc,kd)$ does give integer $a,$ also $gcd(a, kb,kc,kd) = 1.$ Also, the algorithm to find $k$ is easy.
                              $$ k = frac{2b^2+2c^2+d^2}{gcd left( 2b^2+2c^2+d^2 ;, ;2b^3+2c^3+d^3 right)} $$



                              In particular, the mistaken triple (6,4,1) must be multiplied by 35 to get $a$ integral.



                              original     6   4   1 mult    35 gives      187       210     140      35


                              =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=



                              jagy@phobeusjunior:~$ ./mse 
                              original 1 1 1 mult 1 gives 1 1 1 1
                              original 1 1 2 mult 2 gives 3 2 2 4
                              original 1 1 3 mult 13 gives 31 13 13 39
                              original 1 1 4 mult 5 gives 17 5 5 20
                              original 1 1 5 mult 29 gives 129 29 29 145
                              original 1 1 6 mult 2 gives 11 2 2 12
                              original 2 1 1 mult 11 gives 19 22 11 11
                              original 2 1 2 mult 7 gives 13 14 7 14
                              original 2 1 3 mult 19 gives 45 38 19 57
                              original 2 1 4 mult 13 gives 41 26 13 52
                              original 2 1 5 mult 35 gives 143 70 35 175
                              original 2 1 6 mult 23 gives 117 46 23 138
                              original 2 2 1 mult 17 gives 33 34 34 17
                              original 2 2 3 mult 25 gives 59 50 50 75
                              original 2 2 5 mult 41 gives 157 82 82 205
                              original 3 1 1 mult 7 gives 19 21 7 7
                              original 3 1 2 mult 3 gives 8 9 3 6
                              original 3 1 3 mult 29 gives 83 87 29 87
                              original 3 1 4 mult 3 gives 10 9 3 12
                              original 3 1 5 mult 45 gives 181 135 45 225
                              original 3 1 6 mult 7 gives 34 21 7 42
                              original 3 2 1 mult 27 gives 71 81 54 27
                              original 3 2 2 mult 5 gives 13 15 10 10
                              original 3 2 3 mult 35 gives 97 105 70 105
                              original 3 2 4 mult 21 gives 67 63 42 84
                              original 3 2 5 mult 17 gives 65 51 34 85
                              original 3 2 6 mult 31 gives 143 93 62 186
                              original 3 3 1 mult 37 gives 109 111 111 37
                              original 3 3 2 mult 10 gives 29 30 30 20
                              original 3 3 4 mult 13 gives 43 39 39 52
                              original 3 3 5 mult 61 gives 233 183 183 305
                              original 4 1 1 mult 35 gives 131 140 35 35
                              original 4 1 2 mult 19 gives 69 76 19 38
                              original 4 1 3 mult 43 gives 157 172 43 129
                              original 4 1 4 mult 25 gives 97 100 25 100
                              original 4 1 5 mult 59 gives 255 236 59 295
                              original 4 1 6 mult 35 gives 173 140 35 210
                              original 4 2 1 mult 41 gives 145 164 82 41
                              original 4 2 3 mult 49 gives 171 196 98 147
                              original 4 2 5 mult 65 gives 269 260 130 325
                              original 4 3 1 mult 17 gives 61 68 51 17
                              original 4 3 2 mult 27 gives 95 108 81 54
                              original 4 3 3 mult 59 gives 209 236 177 177
                              original 4 3 4 mult 11 gives 41 44 33 44
                              original 4 3 5 mult 75 gives 307 300 225 375
                              original 4 3 6 mult 43 gives 199 172 129 258
                              original 4 4 1 mult 65 gives 257 260 260 65
                              original 4 4 3 mult 73 gives 283 292 292 219
                              original 4 4 5 mult 89 gives 381 356 356 445
                              original 5 1 1 mult 53 gives 253 265 53 53
                              original 5 1 2 mult 14 gives 65 70 14 28
                              original 5 1 3 mult 61 gives 279 305 61 183
                              original 5 1 4 mult 17 gives 79 85 17 68
                              original 5 1 5 mult 77 gives 377 385 77 385
                              original 5 1 6 mult 22 gives 117 110 22 132
                              original 5 2 1 mult 59 gives 267 295 118 59
                              original 5 2 2 mult 31 gives 137 155 62 62
                              original 5 2 3 mult 67 gives 293 335 134 201
                              original 5 2 4 mult 37 gives 165 185 74 148
                              original 5 2 5 mult 83 gives 391 415 166 415
                              original 5 2 6 mult 47 gives 241 235 94 282
                              original 5 3 1 mult 69 gives 305 345 207 69
                              original 5 3 2 mult 3 gives 13 15 9 6
                              original 5 3 3 mult 77 gives 331 385 231 231
                              original 5 3 4 mult 21 gives 92 105 63 84
                              original 5 3 5 mult 31 gives 143 155 93 155
                              original 5 3 6 mult 1 gives 5 5 3 6
                              original 5 4 1 mult 83 gives 379 415 332 83
                              original 5 4 2 mult 43 gives 193 215 172 86
                              original 5 4 3 mult 91 gives 405 455 364 273
                              original 5 4 4 mult 49 gives 221 245 196 196
                              original 5 4 5 mult 107 gives 503 535 428 535
                              original 5 4 6 mult 59 gives 297 295 236 354
                              original 5 5 1 mult 101 gives 501 505 505 101
                              original 5 5 2 mult 26 gives 127 130 130 52
                              original 5 5 3 mult 109 gives 527 545 545 327
                              original 5 5 4 mult 29 gives 141 145 145 116
                              original 5 5 6 mult 34 gives 179 170 170 204
                              original 6 1 1 mult 5 gives 29 30 5 5
                              original 6 1 2 mult 3 gives 17 18 3 6
                              original 6 1 3 mult 83 gives 461 498 83 249
                              original 6 1 4 mult 15 gives 83 90 15 60
                              original 6 1 5 mult 99 gives 559 594 99 495
                              original 6 1 6 mult 11 gives 65 66 11 66
                              original 6 2 1 mult 81 gives 449 486 162 81
                              original 6 2 3 mult 89 gives 475 534 178 267
                              original 6 2 5 mult 35 gives 191 210 70 175
                              original 6 3 1 mult 91 gives 487 546 273 91
                              original 6 3 2 mult 47 gives 247 282 141 94
                              original 6 3 4 mult 53 gives 275 318 159 212
                              original 6 3 5 mult 115 gives 611 690 345 575
                              original 6 4 1 mult 35 gives 187 210 140 35
                              original 6 4 3 mult 113 gives 587 678 452 339
                              original 6 4 5 mult 129 gives 685 774 516 645
                              original 6 5 1 mult 123 gives 683 738 615 123
                              original 6 5 2 mult 21 gives 115 126 105 42
                              original 6 5 3 mult 131 gives 709 786 655 393
                              original 6 5 4 mult 69 gives 373 414 345 276
                              original 6 5 5 mult 49 gives 269 294 245 245
                              original 6 5 6 mult 79 gives 449 474 395 474
                              original 6 6 1 mult 29 gives 173 174 174 29
                              original 6 6 5 mult 169 gives 989 1014 1014 845






                              share|cite|improve this answer














                              share|cite|improve this answer



                              share|cite|improve this answer








                              edited Jan 5 at 20:35

























                              answered Jan 5 at 19:12









                              Will JagyWill Jagy

                              104k5102201




                              104k5102201























                                  0












                                  $begingroup$

                                  For the equation.



                                  $$a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$$



                                  You can record this parameterization.



                                  $$a=p^3-3s^3$$



                                  $$b=p^3+sp^2-9s^3$$



                                  $$c=p(p^2-6s^2)$$



                                  $$d=p^3-2sp^2+9s^3$$






                                  share|cite|improve this answer









                                  $endgroup$


















                                    0












                                    $begingroup$

                                    For the equation.



                                    $$a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$$



                                    You can record this parameterization.



                                    $$a=p^3-3s^3$$



                                    $$b=p^3+sp^2-9s^3$$



                                    $$c=p(p^2-6s^2)$$



                                    $$d=p^3-2sp^2+9s^3$$






                                    share|cite|improve this answer









                                    $endgroup$
















                                      0












                                      0








                                      0





                                      $begingroup$

                                      For the equation.



                                      $$a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$$



                                      You can record this parameterization.



                                      $$a=p^3-3s^3$$



                                      $$b=p^3+sp^2-9s^3$$



                                      $$c=p(p^2-6s^2)$$



                                      $$d=p^3-2sp^2+9s^3$$






                                      share|cite|improve this answer









                                      $endgroup$



                                      For the equation.



                                      $$a=frac{2b^3+2c^3+d^3}{2b^2+2c^2+d^2}$$



                                      You can record this parameterization.



                                      $$a=p^3-3s^3$$



                                      $$b=p^3+sp^2-9s^3$$



                                      $$c=p(p^2-6s^2)$$



                                      $$d=p^3-2sp^2+9s^3$$







                                      share|cite|improve this answer












                                      share|cite|improve this answer



                                      share|cite|improve this answer










                                      answered Jan 6 at 9:03









                                      individindivid

                                      3,2621916




                                      3,2621916






























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