exact-closed form solutions of recurrences
How can I find the exact-closed form solution of this recurrence?
$$f(n) = (1/2)f(n-2) + n/2,$$
where n is even, $f(0)=1$
My answer:
I try to guess a solution by unwinding the formula for f(n)
$f(n) = (1/2)f(n-2) + n/2$$
$= (1/2) ((1/2)f(n-4) + ((n-2)/2) )+n/2$
$=(1/2)^2f(n-4)+n/2^2+n/2 -2/2^2$
$=(1/2)^3f(n-6)+n/2^3+n/2^2+n/2 -2/2^2-4/2^3$
$=(1/2)^4f(n-8)+n/2^4+n/2^3+n/2^2+n/2 -2/2^2-4/2^3-6/2^4$
Continuing in this manner suggest the sum:
$f(n)=1/2^{(n/2)}f(0)+n(1/2^{n/2}+1/2^{(n/2)-1}+...+1/2)-(2/2^2+4/2^3+6/2^4+...)$
$f(n)=1/2^{(n/2)}+n(1-(1/2)^{n/2})-(2/2^2+4/2^3+6/2^4+...)$
How could I solve the expression $(2/2^2+4/2^3+6/2^4+...)$ and find the exact-closed form solution of this recurrence?
discrete-mathematics recurrence-relations
add a comment |
How can I find the exact-closed form solution of this recurrence?
$$f(n) = (1/2)f(n-2) + n/2,$$
where n is even, $f(0)=1$
My answer:
I try to guess a solution by unwinding the formula for f(n)
$f(n) = (1/2)f(n-2) + n/2$$
$= (1/2) ((1/2)f(n-4) + ((n-2)/2) )+n/2$
$=(1/2)^2f(n-4)+n/2^2+n/2 -2/2^2$
$=(1/2)^3f(n-6)+n/2^3+n/2^2+n/2 -2/2^2-4/2^3$
$=(1/2)^4f(n-8)+n/2^4+n/2^3+n/2^2+n/2 -2/2^2-4/2^3-6/2^4$
Continuing in this manner suggest the sum:
$f(n)=1/2^{(n/2)}f(0)+n(1/2^{n/2}+1/2^{(n/2)-1}+...+1/2)-(2/2^2+4/2^3+6/2^4+...)$
$f(n)=1/2^{(n/2)}+n(1-(1/2)^{n/2})-(2/2^2+4/2^3+6/2^4+...)$
How could I solve the expression $(2/2^2+4/2^3+6/2^4+...)$ and find the exact-closed form solution of this recurrence?
discrete-mathematics recurrence-relations
add a comment |
How can I find the exact-closed form solution of this recurrence?
$$f(n) = (1/2)f(n-2) + n/2,$$
where n is even, $f(0)=1$
My answer:
I try to guess a solution by unwinding the formula for f(n)
$f(n) = (1/2)f(n-2) + n/2$$
$= (1/2) ((1/2)f(n-4) + ((n-2)/2) )+n/2$
$=(1/2)^2f(n-4)+n/2^2+n/2 -2/2^2$
$=(1/2)^3f(n-6)+n/2^3+n/2^2+n/2 -2/2^2-4/2^3$
$=(1/2)^4f(n-8)+n/2^4+n/2^3+n/2^2+n/2 -2/2^2-4/2^3-6/2^4$
Continuing in this manner suggest the sum:
$f(n)=1/2^{(n/2)}f(0)+n(1/2^{n/2}+1/2^{(n/2)-1}+...+1/2)-(2/2^2+4/2^3+6/2^4+...)$
$f(n)=1/2^{(n/2)}+n(1-(1/2)^{n/2})-(2/2^2+4/2^3+6/2^4+...)$
How could I solve the expression $(2/2^2+4/2^3+6/2^4+...)$ and find the exact-closed form solution of this recurrence?
discrete-mathematics recurrence-relations
How can I find the exact-closed form solution of this recurrence?
$$f(n) = (1/2)f(n-2) + n/2,$$
where n is even, $f(0)=1$
My answer:
I try to guess a solution by unwinding the formula for f(n)
$f(n) = (1/2)f(n-2) + n/2$$
$= (1/2) ((1/2)f(n-4) + ((n-2)/2) )+n/2$
$=(1/2)^2f(n-4)+n/2^2+n/2 -2/2^2$
$=(1/2)^3f(n-6)+n/2^3+n/2^2+n/2 -2/2^2-4/2^3$
$=(1/2)^4f(n-8)+n/2^4+n/2^3+n/2^2+n/2 -2/2^2-4/2^3-6/2^4$
Continuing in this manner suggest the sum:
$f(n)=1/2^{(n/2)}f(0)+n(1/2^{n/2}+1/2^{(n/2)-1}+...+1/2)-(2/2^2+4/2^3+6/2^4+...)$
$f(n)=1/2^{(n/2)}+n(1-(1/2)^{n/2})-(2/2^2+4/2^3+6/2^4+...)$
How could I solve the expression $(2/2^2+4/2^3+6/2^4+...)$ and find the exact-closed form solution of this recurrence?
discrete-mathematics recurrence-relations
discrete-mathematics recurrence-relations
asked Nov 26 '18 at 14:28
user614642
278
278
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3 Answers
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Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$
Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
– user614642
Nov 26 '18 at 15:26
add a comment |
Hint
Let $g(m)=f(m)+a_0+a_1m+cdots$
$n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$
Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$
so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$
Set $2r=n$
I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
– user614642
Nov 27 '18 at 3:36
add a comment |
Your direct question is about the sum
$$
eqalign{
& 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
& = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
& = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
$$
Since
$$
eqalign{
& sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
= z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
& = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
$$
then the sum equals
$$
sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
$$
Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
– user614642
Nov 27 '18 at 3:26
add a comment |
Your Answer
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3 Answers
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3 Answers
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Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$
Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
– user614642
Nov 26 '18 at 15:26
add a comment |
Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$
Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
– user614642
Nov 26 '18 at 15:26
add a comment |
Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$
Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$
answered Nov 26 '18 at 14:37
Robert Israel
318k23208457
318k23208457
Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
– user614642
Nov 26 '18 at 15:26
add a comment |
Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
– user614642
Nov 26 '18 at 15:26
Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
– user614642
Nov 26 '18 at 15:26
Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
– user614642
Nov 26 '18 at 15:26
add a comment |
Hint
Let $g(m)=f(m)+a_0+a_1m+cdots$
$n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$
Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$
so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$
Set $2r=n$
I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
– user614642
Nov 27 '18 at 3:36
add a comment |
Hint
Let $g(m)=f(m)+a_0+a_1m+cdots$
$n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$
Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$
so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$
Set $2r=n$
I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
– user614642
Nov 27 '18 at 3:36
add a comment |
Hint
Let $g(m)=f(m)+a_0+a_1m+cdots$
$n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$
Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$
so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$
Set $2r=n$
Hint
Let $g(m)=f(m)+a_0+a_1m+cdots$
$n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$
Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$
so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$
Set $2r=n$
answered Nov 26 '18 at 14:50
lab bhattacharjee
223k15156274
223k15156274
I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
– user614642
Nov 27 '18 at 3:36
add a comment |
I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
– user614642
Nov 27 '18 at 3:36
I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
– user614642
Nov 27 '18 at 3:36
I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
– user614642
Nov 27 '18 at 3:36
add a comment |
Your direct question is about the sum
$$
eqalign{
& 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
& = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
& = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
$$
Since
$$
eqalign{
& sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
= z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
& = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
$$
then the sum equals
$$
sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
$$
Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
– user614642
Nov 27 '18 at 3:26
add a comment |
Your direct question is about the sum
$$
eqalign{
& 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
& = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
& = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
$$
Since
$$
eqalign{
& sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
= z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
& = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
$$
then the sum equals
$$
sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
$$
Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
– user614642
Nov 27 '18 at 3:26
add a comment |
Your direct question is about the sum
$$
eqalign{
& 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
& = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
& = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
$$
Since
$$
eqalign{
& sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
= z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
& = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
$$
then the sum equals
$$
sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
$$
Your direct question is about the sum
$$
eqalign{
& 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
& = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
& = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
$$
Since
$$
eqalign{
& sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
= z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
& = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
$$
then the sum equals
$$
sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
$$
answered Nov 26 '18 at 16:57
G Cab
18k31237
18k31237
Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
– user614642
Nov 27 '18 at 3:26
add a comment |
Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
– user614642
Nov 27 '18 at 3:26
Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
– user614642
Nov 27 '18 at 3:26
Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
– user614642
Nov 27 '18 at 3:26
add a comment |
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