$u_{n+1} = a u_n +b u_{n-1} +c$
$begingroup$
Is there a closed form formula for the general term of a recurrence sequence satisfying $$u_{n+1} = a u_n +b u_{n-1} +c$$ The sequence I am interested in satisfies $u_{n+1} = 8 u_n - 3 u_{n-1} -4$.
real-analysis sequences-and-series recurrence-relations
$endgroup$
add a comment |
$begingroup$
Is there a closed form formula for the general term of a recurrence sequence satisfying $$u_{n+1} = a u_n +b u_{n-1} +c$$ The sequence I am interested in satisfies $u_{n+1} = 8 u_n - 3 u_{n-1} -4$.
real-analysis sequences-and-series recurrence-relations
$endgroup$
add a comment |
$begingroup$
Is there a closed form formula for the general term of a recurrence sequence satisfying $$u_{n+1} = a u_n +b u_{n-1} +c$$ The sequence I am interested in satisfies $u_{n+1} = 8 u_n - 3 u_{n-1} -4$.
real-analysis sequences-and-series recurrence-relations
$endgroup$
Is there a closed form formula for the general term of a recurrence sequence satisfying $$u_{n+1} = a u_n +b u_{n-1} +c$$ The sequence I am interested in satisfies $u_{n+1} = 8 u_n - 3 u_{n-1} -4$.
real-analysis sequences-and-series recurrence-relations
real-analysis sequences-and-series recurrence-relations
edited Dec 20 '18 at 15:04
Ankit Kumar
1,384220
1,384220
asked Dec 20 '18 at 13:59
furyofuryo
263
263
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
You can solve any non-homogeneous, linear recurrence of the above form by converting it to homogeneous form.
(I'll use $n$ instead of $n+1$, doesn't matter anyway!)
$$u_{n}=au_{n-1}+bu_{n-2}+c$$
$$u_{n-1}=au_{n-2}+bu_{n-3}+c$$
Subtract them to get:-
$$u_{n}=(a+1)u_{n-1}+(b-a)u_{n-2}-bu_{n-3}$$
You can then use characteristic polynomial to find the answer.
Speaking strictly for the equation you wrote, the characteristic polynomial is
$$z^3=9z^2-11z+3$$ which has the roots
$$z=1, 4+sqrt{13}, 4-sqrt{13}$$
giving $$u_n=a_1+a_2{(4+sqrt{13})}^n+a_3{(4-sqrt{13})}^n$$, where $a_i$s depend on the initial condition.
$endgroup$
$begingroup$
So $u_n$ wouldn't converge? I find it a little bit strange.
$endgroup$
– furyo
Dec 20 '18 at 16:39
$begingroup$
@furyo It isn't strange at all. Stuff like this (convergence) depends on the initial conditions. Depending upon them, the series may or may not convegre!
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:43
$begingroup$
For instance, if you take $u_1=u_2=1$, $u_i=1$ for all $i$, which is a convergent sequence. On the other hand, if you take $u_1=1, u_2=2$, then it'll be a divergent sequence.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:44
$begingroup$
I would've expected a non constant sequence increasing to $1$. Here it's either constant or divergent.
$endgroup$
– furyo
Dec 20 '18 at 16:47
$begingroup$
You can't get a non-constant, convergent sequence because it isn't constant $implies $ at least one of $a_2, a_3$ is non-zero $implies $ $u_n$ will be a strictly increasing/decreasing (depending upon sign of $a_i$s) function of $n$.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:54
add a comment |
$begingroup$
This is a linear difference equation so can be stated as
$$
u = u_h + u_p
$$
with
$$
u_h(n+1)-au_h(n)-bu_h(n-1) = 0\
u_p(n+1)-au_p(n)-bu_p(n-1) = c\
$$
now making $u_h(n) = phi^n$ and substituting we have
$$
phi^nleft(phi-a-bphi^{-1}right) = 0
$$
so
$$
phi = frac{1}{2} left(apmsqrt{a^2+4 b}right)
$$
and
$$
u_h(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n
$$
and also
$$
u_p(n) = frac{c}{1-a-b}
$$
so finally
$$
u(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n + frac{c}{1-a-b}
$$
$endgroup$
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
You can solve any non-homogeneous, linear recurrence of the above form by converting it to homogeneous form.
(I'll use $n$ instead of $n+1$, doesn't matter anyway!)
$$u_{n}=au_{n-1}+bu_{n-2}+c$$
$$u_{n-1}=au_{n-2}+bu_{n-3}+c$$
Subtract them to get:-
$$u_{n}=(a+1)u_{n-1}+(b-a)u_{n-2}-bu_{n-3}$$
You can then use characteristic polynomial to find the answer.
Speaking strictly for the equation you wrote, the characteristic polynomial is
$$z^3=9z^2-11z+3$$ which has the roots
$$z=1, 4+sqrt{13}, 4-sqrt{13}$$
giving $$u_n=a_1+a_2{(4+sqrt{13})}^n+a_3{(4-sqrt{13})}^n$$, where $a_i$s depend on the initial condition.
$endgroup$
$begingroup$
So $u_n$ wouldn't converge? I find it a little bit strange.
$endgroup$
– furyo
Dec 20 '18 at 16:39
$begingroup$
@furyo It isn't strange at all. Stuff like this (convergence) depends on the initial conditions. Depending upon them, the series may or may not convegre!
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:43
$begingroup$
For instance, if you take $u_1=u_2=1$, $u_i=1$ for all $i$, which is a convergent sequence. On the other hand, if you take $u_1=1, u_2=2$, then it'll be a divergent sequence.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:44
$begingroup$
I would've expected a non constant sequence increasing to $1$. Here it's either constant or divergent.
$endgroup$
– furyo
Dec 20 '18 at 16:47
$begingroup$
You can't get a non-constant, convergent sequence because it isn't constant $implies $ at least one of $a_2, a_3$ is non-zero $implies $ $u_n$ will be a strictly increasing/decreasing (depending upon sign of $a_i$s) function of $n$.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:54
add a comment |
$begingroup$
You can solve any non-homogeneous, linear recurrence of the above form by converting it to homogeneous form.
(I'll use $n$ instead of $n+1$, doesn't matter anyway!)
$$u_{n}=au_{n-1}+bu_{n-2}+c$$
$$u_{n-1}=au_{n-2}+bu_{n-3}+c$$
Subtract them to get:-
$$u_{n}=(a+1)u_{n-1}+(b-a)u_{n-2}-bu_{n-3}$$
You can then use characteristic polynomial to find the answer.
Speaking strictly for the equation you wrote, the characteristic polynomial is
$$z^3=9z^2-11z+3$$ which has the roots
$$z=1, 4+sqrt{13}, 4-sqrt{13}$$
giving $$u_n=a_1+a_2{(4+sqrt{13})}^n+a_3{(4-sqrt{13})}^n$$, where $a_i$s depend on the initial condition.
$endgroup$
$begingroup$
So $u_n$ wouldn't converge? I find it a little bit strange.
$endgroup$
– furyo
Dec 20 '18 at 16:39
$begingroup$
@furyo It isn't strange at all. Stuff like this (convergence) depends on the initial conditions. Depending upon them, the series may or may not convegre!
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:43
$begingroup$
For instance, if you take $u_1=u_2=1$, $u_i=1$ for all $i$, which is a convergent sequence. On the other hand, if you take $u_1=1, u_2=2$, then it'll be a divergent sequence.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:44
$begingroup$
I would've expected a non constant sequence increasing to $1$. Here it's either constant or divergent.
$endgroup$
– furyo
Dec 20 '18 at 16:47
$begingroup$
You can't get a non-constant, convergent sequence because it isn't constant $implies $ at least one of $a_2, a_3$ is non-zero $implies $ $u_n$ will be a strictly increasing/decreasing (depending upon sign of $a_i$s) function of $n$.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:54
add a comment |
$begingroup$
You can solve any non-homogeneous, linear recurrence of the above form by converting it to homogeneous form.
(I'll use $n$ instead of $n+1$, doesn't matter anyway!)
$$u_{n}=au_{n-1}+bu_{n-2}+c$$
$$u_{n-1}=au_{n-2}+bu_{n-3}+c$$
Subtract them to get:-
$$u_{n}=(a+1)u_{n-1}+(b-a)u_{n-2}-bu_{n-3}$$
You can then use characteristic polynomial to find the answer.
Speaking strictly for the equation you wrote, the characteristic polynomial is
$$z^3=9z^2-11z+3$$ which has the roots
$$z=1, 4+sqrt{13}, 4-sqrt{13}$$
giving $$u_n=a_1+a_2{(4+sqrt{13})}^n+a_3{(4-sqrt{13})}^n$$, where $a_i$s depend on the initial condition.
$endgroup$
You can solve any non-homogeneous, linear recurrence of the above form by converting it to homogeneous form.
(I'll use $n$ instead of $n+1$, doesn't matter anyway!)
$$u_{n}=au_{n-1}+bu_{n-2}+c$$
$$u_{n-1}=au_{n-2}+bu_{n-3}+c$$
Subtract them to get:-
$$u_{n}=(a+1)u_{n-1}+(b-a)u_{n-2}-bu_{n-3}$$
You can then use characteristic polynomial to find the answer.
Speaking strictly for the equation you wrote, the characteristic polynomial is
$$z^3=9z^2-11z+3$$ which has the roots
$$z=1, 4+sqrt{13}, 4-sqrt{13}$$
giving $$u_n=a_1+a_2{(4+sqrt{13})}^n+a_3{(4-sqrt{13})}^n$$, where $a_i$s depend on the initial condition.
answered Dec 20 '18 at 14:24
Ankit KumarAnkit Kumar
1,384220
1,384220
$begingroup$
So $u_n$ wouldn't converge? I find it a little bit strange.
$endgroup$
– furyo
Dec 20 '18 at 16:39
$begingroup$
@furyo It isn't strange at all. Stuff like this (convergence) depends on the initial conditions. Depending upon them, the series may or may not convegre!
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:43
$begingroup$
For instance, if you take $u_1=u_2=1$, $u_i=1$ for all $i$, which is a convergent sequence. On the other hand, if you take $u_1=1, u_2=2$, then it'll be a divergent sequence.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:44
$begingroup$
I would've expected a non constant sequence increasing to $1$. Here it's either constant or divergent.
$endgroup$
– furyo
Dec 20 '18 at 16:47
$begingroup$
You can't get a non-constant, convergent sequence because it isn't constant $implies $ at least one of $a_2, a_3$ is non-zero $implies $ $u_n$ will be a strictly increasing/decreasing (depending upon sign of $a_i$s) function of $n$.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:54
add a comment |
$begingroup$
So $u_n$ wouldn't converge? I find it a little bit strange.
$endgroup$
– furyo
Dec 20 '18 at 16:39
$begingroup$
@furyo It isn't strange at all. Stuff like this (convergence) depends on the initial conditions. Depending upon them, the series may or may not convegre!
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:43
$begingroup$
For instance, if you take $u_1=u_2=1$, $u_i=1$ for all $i$, which is a convergent sequence. On the other hand, if you take $u_1=1, u_2=2$, then it'll be a divergent sequence.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:44
$begingroup$
I would've expected a non constant sequence increasing to $1$. Here it's either constant or divergent.
$endgroup$
– furyo
Dec 20 '18 at 16:47
$begingroup$
You can't get a non-constant, convergent sequence because it isn't constant $implies $ at least one of $a_2, a_3$ is non-zero $implies $ $u_n$ will be a strictly increasing/decreasing (depending upon sign of $a_i$s) function of $n$.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:54
$begingroup$
So $u_n$ wouldn't converge? I find it a little bit strange.
$endgroup$
– furyo
Dec 20 '18 at 16:39
$begingroup$
So $u_n$ wouldn't converge? I find it a little bit strange.
$endgroup$
– furyo
Dec 20 '18 at 16:39
$begingroup$
@furyo It isn't strange at all. Stuff like this (convergence) depends on the initial conditions. Depending upon them, the series may or may not convegre!
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:43
$begingroup$
@furyo It isn't strange at all. Stuff like this (convergence) depends on the initial conditions. Depending upon them, the series may or may not convegre!
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:43
$begingroup$
For instance, if you take $u_1=u_2=1$, $u_i=1$ for all $i$, which is a convergent sequence. On the other hand, if you take $u_1=1, u_2=2$, then it'll be a divergent sequence.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:44
$begingroup$
For instance, if you take $u_1=u_2=1$, $u_i=1$ for all $i$, which is a convergent sequence. On the other hand, if you take $u_1=1, u_2=2$, then it'll be a divergent sequence.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:44
$begingroup$
I would've expected a non constant sequence increasing to $1$. Here it's either constant or divergent.
$endgroup$
– furyo
Dec 20 '18 at 16:47
$begingroup$
I would've expected a non constant sequence increasing to $1$. Here it's either constant or divergent.
$endgroup$
– furyo
Dec 20 '18 at 16:47
$begingroup$
You can't get a non-constant, convergent sequence because it isn't constant $implies $ at least one of $a_2, a_3$ is non-zero $implies $ $u_n$ will be a strictly increasing/decreasing (depending upon sign of $a_i$s) function of $n$.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:54
$begingroup$
You can't get a non-constant, convergent sequence because it isn't constant $implies $ at least one of $a_2, a_3$ is non-zero $implies $ $u_n$ will be a strictly increasing/decreasing (depending upon sign of $a_i$s) function of $n$.
$endgroup$
– Ankit Kumar
Dec 20 '18 at 16:54
add a comment |
$begingroup$
This is a linear difference equation so can be stated as
$$
u = u_h + u_p
$$
with
$$
u_h(n+1)-au_h(n)-bu_h(n-1) = 0\
u_p(n+1)-au_p(n)-bu_p(n-1) = c\
$$
now making $u_h(n) = phi^n$ and substituting we have
$$
phi^nleft(phi-a-bphi^{-1}right) = 0
$$
so
$$
phi = frac{1}{2} left(apmsqrt{a^2+4 b}right)
$$
and
$$
u_h(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n
$$
and also
$$
u_p(n) = frac{c}{1-a-b}
$$
so finally
$$
u(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n + frac{c}{1-a-b}
$$
$endgroup$
add a comment |
$begingroup$
This is a linear difference equation so can be stated as
$$
u = u_h + u_p
$$
with
$$
u_h(n+1)-au_h(n)-bu_h(n-1) = 0\
u_p(n+1)-au_p(n)-bu_p(n-1) = c\
$$
now making $u_h(n) = phi^n$ and substituting we have
$$
phi^nleft(phi-a-bphi^{-1}right) = 0
$$
so
$$
phi = frac{1}{2} left(apmsqrt{a^2+4 b}right)
$$
and
$$
u_h(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n
$$
and also
$$
u_p(n) = frac{c}{1-a-b}
$$
so finally
$$
u(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n + frac{c}{1-a-b}
$$
$endgroup$
add a comment |
$begingroup$
This is a linear difference equation so can be stated as
$$
u = u_h + u_p
$$
with
$$
u_h(n+1)-au_h(n)-bu_h(n-1) = 0\
u_p(n+1)-au_p(n)-bu_p(n-1) = c\
$$
now making $u_h(n) = phi^n$ and substituting we have
$$
phi^nleft(phi-a-bphi^{-1}right) = 0
$$
so
$$
phi = frac{1}{2} left(apmsqrt{a^2+4 b}right)
$$
and
$$
u_h(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n
$$
and also
$$
u_p(n) = frac{c}{1-a-b}
$$
so finally
$$
u(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n + frac{c}{1-a-b}
$$
$endgroup$
This is a linear difference equation so can be stated as
$$
u = u_h + u_p
$$
with
$$
u_h(n+1)-au_h(n)-bu_h(n-1) = 0\
u_p(n+1)-au_p(n)-bu_p(n-1) = c\
$$
now making $u_h(n) = phi^n$ and substituting we have
$$
phi^nleft(phi-a-bphi^{-1}right) = 0
$$
so
$$
phi = frac{1}{2} left(apmsqrt{a^2+4 b}right)
$$
and
$$
u_h(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n
$$
and also
$$
u_p(n) = frac{c}{1-a-b}
$$
so finally
$$
u(n) = frac{C_1}{2^n} left(a-sqrt{a^2+4 b}right)^n+frac{C_2}{2^n} left(a+sqrt{a^2+4 b}right)^n + frac{c}{1-a-b}
$$
answered Dec 20 '18 at 14:25
CesareoCesareo
8,6743516
8,6743516
add a comment |
add a comment |
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