$int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x = 0$?
$begingroup$
Does
$$int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x = 0$$
for all real numbers $a > 0$ and $b < 2$?
I came across this conjecture by showing its validity for the positive integer values of $a$ only.
To derive the result for positive integer $a$, make the substitution $u=frac1x$ on
$$ int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x $$
and we get
$$ int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x \
implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0
$$
From here, we can use Leibniz's rule of integration (differentiating with respect to $b$) $n$ times to retrieve
$$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x = 0$$
Yet I'm guessing that it is also valid for any real positive value of $a$ from numerical evidence. Complex methods are welcome but I won't really be familiar with them, so I would prefer sticking to real methods.
calculus integration definite-integrals improper-integrals
$endgroup$
add a comment |
$begingroup$
Does
$$int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x = 0$$
for all real numbers $a > 0$ and $b < 2$?
I came across this conjecture by showing its validity for the positive integer values of $a$ only.
To derive the result for positive integer $a$, make the substitution $u=frac1x$ on
$$ int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x $$
and we get
$$ int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x \
implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0
$$
From here, we can use Leibniz's rule of integration (differentiating with respect to $b$) $n$ times to retrieve
$$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x = 0$$
Yet I'm guessing that it is also valid for any real positive value of $a$ from numerical evidence. Complex methods are welcome but I won't really be familiar with them, so I would prefer sticking to real methods.
calculus integration definite-integrals improper-integrals
$endgroup$
$begingroup$
I am not quite sure: How do you get from $$int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x$$ to $$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x$$? With respect to which parameter do you differentiate?
$endgroup$
– mrtaurho
Dec 30 '18 at 14:40
$begingroup$
Differentiate with respect to $b$.
$endgroup$
– Mint
Dec 30 '18 at 14:41
1
$begingroup$
I would include this detail hence, at least to me, it was not clear at all.
$endgroup$
– mrtaurho
Dec 30 '18 at 14:45
1
$begingroup$
Same proof as before.
$endgroup$
– Jack D'Aurizio
Dec 30 '18 at 15:00
add a comment |
$begingroup$
Does
$$int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x = 0$$
for all real numbers $a > 0$ and $b < 2$?
I came across this conjecture by showing its validity for the positive integer values of $a$ only.
To derive the result for positive integer $a$, make the substitution $u=frac1x$ on
$$ int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x $$
and we get
$$ int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x \
implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0
$$
From here, we can use Leibniz's rule of integration (differentiating with respect to $b$) $n$ times to retrieve
$$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x = 0$$
Yet I'm guessing that it is also valid for any real positive value of $a$ from numerical evidence. Complex methods are welcome but I won't really be familiar with them, so I would prefer sticking to real methods.
calculus integration definite-integrals improper-integrals
$endgroup$
Does
$$int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x = 0$$
for all real numbers $a > 0$ and $b < 2$?
I came across this conjecture by showing its validity for the positive integer values of $a$ only.
To derive the result for positive integer $a$, make the substitution $u=frac1x$ on
$$ int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x $$
and we get
$$ int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x \
implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0
$$
From here, we can use Leibniz's rule of integration (differentiating with respect to $b$) $n$ times to retrieve
$$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x = 0$$
Yet I'm guessing that it is also valid for any real positive value of $a$ from numerical evidence. Complex methods are welcome but I won't really be familiar with them, so I would prefer sticking to real methods.
calculus integration definite-integrals improper-integrals
calculus integration definite-integrals improper-integrals
edited Dec 30 '18 at 14:56
Mint
asked Dec 30 '18 at 14:35
MintMint
5311417
5311417
$begingroup$
I am not quite sure: How do you get from $$int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x$$ to $$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x$$? With respect to which parameter do you differentiate?
$endgroup$
– mrtaurho
Dec 30 '18 at 14:40
$begingroup$
Differentiate with respect to $b$.
$endgroup$
– Mint
Dec 30 '18 at 14:41
1
$begingroup$
I would include this detail hence, at least to me, it was not clear at all.
$endgroup$
– mrtaurho
Dec 30 '18 at 14:45
1
$begingroup$
Same proof as before.
$endgroup$
– Jack D'Aurizio
Dec 30 '18 at 15:00
add a comment |
$begingroup$
I am not quite sure: How do you get from $$int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x$$ to $$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x$$? With respect to which parameter do you differentiate?
$endgroup$
– mrtaurho
Dec 30 '18 at 14:40
$begingroup$
Differentiate with respect to $b$.
$endgroup$
– Mint
Dec 30 '18 at 14:41
1
$begingroup$
I would include this detail hence, at least to me, it was not clear at all.
$endgroup$
– mrtaurho
Dec 30 '18 at 14:45
1
$begingroup$
Same proof as before.
$endgroup$
– Jack D'Aurizio
Dec 30 '18 at 15:00
$begingroup$
I am not quite sure: How do you get from $$int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x$$ to $$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x$$? With respect to which parameter do you differentiate?
$endgroup$
– mrtaurho
Dec 30 '18 at 14:40
$begingroup$
I am not quite sure: How do you get from $$int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x$$ to $$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x$$? With respect to which parameter do you differentiate?
$endgroup$
– mrtaurho
Dec 30 '18 at 14:40
$begingroup$
Differentiate with respect to $b$.
$endgroup$
– Mint
Dec 30 '18 at 14:41
$begingroup$
Differentiate with respect to $b$.
$endgroup$
– Mint
Dec 30 '18 at 14:41
1
1
$begingroup$
I would include this detail hence, at least to me, it was not clear at all.
$endgroup$
– mrtaurho
Dec 30 '18 at 14:45
$begingroup$
I would include this detail hence, at least to me, it was not clear at all.
$endgroup$
– mrtaurho
Dec 30 '18 at 14:45
1
1
$begingroup$
Same proof as before.
$endgroup$
– Jack D'Aurizio
Dec 30 '18 at 15:00
$begingroup$
Same proof as before.
$endgroup$
– Jack D'Aurizio
Dec 30 '18 at 15:00
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
Of course it does. Just substitute $displaystyle{x=frac{1}{t}}$ to get:
$$I=int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x=int_infty^0 frac{1}{t^a} frac{lnleft(frac{1}{t}right)}{left(frac{1}{t^2}-frac{b}{t}+1right)^{a+1}}frac{-dt}{t^2}$$
$$=int_0^infty frac{t^{2a+2}lnleft(frac{1}{t}right)}{left(1-bt+t^2right)^{a+1}}frac{dt}{t^{a+2}}=int_0^infty frac{t^alnleft(frac{1}{t}right)}{(t^2-bt+1)^{a+1}}dt=-I$$
$$I=-IRightarrow I=0$$
$endgroup$
$begingroup$
Omg I feel so dumb, thank you so much. Standard substitutions slip by me occasionally...
$endgroup$
– Mint
Dec 30 '18 at 14:51
2
$begingroup$
I like your derivation in your question! A small typo:$$int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - color{red}k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0$$
$endgroup$
– Zacky
Dec 30 '18 at 14:53
add a comment |
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1 Answer
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$begingroup$
Of course it does. Just substitute $displaystyle{x=frac{1}{t}}$ to get:
$$I=int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x=int_infty^0 frac{1}{t^a} frac{lnleft(frac{1}{t}right)}{left(frac{1}{t^2}-frac{b}{t}+1right)^{a+1}}frac{-dt}{t^2}$$
$$=int_0^infty frac{t^{2a+2}lnleft(frac{1}{t}right)}{left(1-bt+t^2right)^{a+1}}frac{dt}{t^{a+2}}=int_0^infty frac{t^alnleft(frac{1}{t}right)}{(t^2-bt+1)^{a+1}}dt=-I$$
$$I=-IRightarrow I=0$$
$endgroup$
$begingroup$
Omg I feel so dumb, thank you so much. Standard substitutions slip by me occasionally...
$endgroup$
– Mint
Dec 30 '18 at 14:51
2
$begingroup$
I like your derivation in your question! A small typo:$$int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - color{red}k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0$$
$endgroup$
– Zacky
Dec 30 '18 at 14:53
add a comment |
$begingroup$
Of course it does. Just substitute $displaystyle{x=frac{1}{t}}$ to get:
$$I=int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x=int_infty^0 frac{1}{t^a} frac{lnleft(frac{1}{t}right)}{left(frac{1}{t^2}-frac{b}{t}+1right)^{a+1}}frac{-dt}{t^2}$$
$$=int_0^infty frac{t^{2a+2}lnleft(frac{1}{t}right)}{left(1-bt+t^2right)^{a+1}}frac{dt}{t^{a+2}}=int_0^infty frac{t^alnleft(frac{1}{t}right)}{(t^2-bt+1)^{a+1}}dt=-I$$
$$I=-IRightarrow I=0$$
$endgroup$
$begingroup$
Omg I feel so dumb, thank you so much. Standard substitutions slip by me occasionally...
$endgroup$
– Mint
Dec 30 '18 at 14:51
2
$begingroup$
I like your derivation in your question! A small typo:$$int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - color{red}k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0$$
$endgroup$
– Zacky
Dec 30 '18 at 14:53
add a comment |
$begingroup$
Of course it does. Just substitute $displaystyle{x=frac{1}{t}}$ to get:
$$I=int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x=int_infty^0 frac{1}{t^a} frac{lnleft(frac{1}{t}right)}{left(frac{1}{t^2}-frac{b}{t}+1right)^{a+1}}frac{-dt}{t^2}$$
$$=int_0^infty frac{t^{2a+2}lnleft(frac{1}{t}right)}{left(1-bt+t^2right)^{a+1}}frac{dt}{t^{a+2}}=int_0^infty frac{t^alnleft(frac{1}{t}right)}{(t^2-bt+1)^{a+1}}dt=-I$$
$$I=-IRightarrow I=0$$
$endgroup$
Of course it does. Just substitute $displaystyle{x=frac{1}{t}}$ to get:
$$I=int_0^infty frac{x^aln(x)}{(x^2-bx+1)^{a+1}}{rm d}x=int_infty^0 frac{1}{t^a} frac{lnleft(frac{1}{t}right)}{left(frac{1}{t^2}-frac{b}{t}+1right)^{a+1}}frac{-dt}{t^2}$$
$$=int_0^infty frac{t^{2a+2}lnleft(frac{1}{t}right)}{left(1-bt+t^2right)^{a+1}}frac{dt}{t^{a+2}}=int_0^infty frac{t^alnleft(frac{1}{t}right)}{(t^2-bt+1)^{a+1}}dt=-I$$
$$I=-IRightarrow I=0$$
edited Dec 30 '18 at 15:02
answered Dec 30 '18 at 14:46
ZackyZacky
7,1501961
7,1501961
$begingroup$
Omg I feel so dumb, thank you so much. Standard substitutions slip by me occasionally...
$endgroup$
– Mint
Dec 30 '18 at 14:51
2
$begingroup$
I like your derivation in your question! A small typo:$$int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - color{red}k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0$$
$endgroup$
– Zacky
Dec 30 '18 at 14:53
add a comment |
$begingroup$
Omg I feel so dumb, thank you so much. Standard substitutions slip by me occasionally...
$endgroup$
– Mint
Dec 30 '18 at 14:51
2
$begingroup$
I like your derivation in your question! A small typo:$$int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - color{red}k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0$$
$endgroup$
– Zacky
Dec 30 '18 at 14:53
$begingroup$
Omg I feel so dumb, thank you so much. Standard substitutions slip by me occasionally...
$endgroup$
– Mint
Dec 30 '18 at 14:51
$begingroup$
Omg I feel so dumb, thank you so much. Standard substitutions slip by me occasionally...
$endgroup$
– Mint
Dec 30 '18 at 14:51
2
2
$begingroup$
I like your derivation in your question! A small typo:$$int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - color{red}k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0$$
$endgroup$
– Zacky
Dec 30 '18 at 14:53
$begingroup$
I like your derivation in your question! A small typo:$$int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x = int_0^infty frac{ln(1+x^k)}{x^2-bx+1}{rm d}x - color{red}k int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x implies int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x = 0$$
$endgroup$
– Zacky
Dec 30 '18 at 14:53
add a comment |
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$begingroup$
I am not quite sure: How do you get from $$int_0^infty frac{ln(x)}{x^2-bx+1}{rm d}x$$ to $$int_0^infty frac{x^nln(x)}{(x^2-bx+1)^{n+1}}{rm d}x$$? With respect to which parameter do you differentiate?
$endgroup$
– mrtaurho
Dec 30 '18 at 14:40
$begingroup$
Differentiate with respect to $b$.
$endgroup$
– Mint
Dec 30 '18 at 14:41
1
$begingroup$
I would include this detail hence, at least to me, it was not clear at all.
$endgroup$
– mrtaurho
Dec 30 '18 at 14:45
1
$begingroup$
Same proof as before.
$endgroup$
– Jack D'Aurizio
Dec 30 '18 at 15:00