Power series representation

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Homework Statement



Find the power series representation for s(x) and s`(x)
integral sin (pi t^2)\2
and which of them is valid ?

Homework Equations





The Attempt at a Solution



I tried to solve this question , but i am not sure
s`(x) = sin (pi t^2)\2 by Abel's theorm

[tex]\sum\ell^in\theta[/tex]

and it converges
I am not sure about the solution , Is it right ? and what about s(x)
Is it - cos(pi t^2)\2 ??? and how we can find the power series representaion for it?
 
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  • #2
SammyS
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Homework Statement



Find the power series representation for s(x) and s`(x)
I take it that you meant:
[tex]s(x)=\int_0^x \sin \left(\frac{\pi t^2}{2}\right)\,dt [/tex]

and which of them is valid ?

Homework Equations





The Attempt at a Solution



I tried to solve this question , but i am not sure
s`(x) = [tex]sin (pi t^2)\2[tex] by Abel's theorm
[tex]\sum [/tex][tex]\ell^ in \theta [/tex]
and it converges
I am not sure about the solution , Is it right ? and what about s(x)
Is it [tex]\ - cos(pi t^2)\2[/tex] ???
[tex]s'(x)=\sin \left(\frac{\pi x^2}{2}\right) [/tex]

Do you know the Taylor Series (actually Maclaurin Series) for sin(x) ?

Substitute πx2/2 for x into the Taylor Series for sin(x).

To find the power series for s(x), integrate the power series for s'(t) from 0 (or whatever the original problem had) to x .
 
  • #3
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[tex]s'(x)=\sin \left(\frac{\pi x^2}{2}\right) [/tex]

Do you know the Taylor Series (actually Maclaurin Series) for sin(x) ?

Substitute πx2/2 for x into the Taylor Series for sin(x).

To find the power series for s(x), integrate the power series for s'(t) from 0 (or whatever the original problem had) to x .
The taylor serries for sin (x) = [tex]\sum\frac{-1^{n}}{(2n+1)!}x^{2n+1}[/tex]

and by substituting (pi x^2)\2
we get

[tex]\sum\frac{-1^{n}}{(2n+1)!}\left(\frac{\pi\times x^{2}}{2}\right)^{2n+1}[/tex]

Woow It is a power series now

s(x) =[tex]\int \sum\frac{-1^{n}}{(2n+1)!}\left(\frac{\pi\times x^{2}}{2}\right)^{2n+1}[/tex]

but how can i check if these two are valid ???


Thanks alot for these useful advices
 
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  • #4
SammyS
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...

s(x) =[tex]\int \sum\frac{-1^{n}}{(2n+1)!}\left(\frac{\pi\times x^{2}}{2}\right)^{2n+1}[/tex]

but how can i check if these two are valid ???

Thanks a lot for these useful advices
You can actually do the integration if each term in the sum.

[tex]s(x)=\int \left(\sum_{n=0}^\infty\frac{(-1)^{n}}{(2n+1)!}\left(\frac{\pi\times x^{2}}{2}\right)^{2n+1}\right)\,dx[/tex]
[tex]=\sum_{n=0}^\infty \left(\int \frac{(-1)^{n}}{(2n+1)!}\left(\frac{\pi\times x^{2}}{2}\right)^{2n+1}\,dx\right)[/tex]

[tex]=\sum_{n=0}^\infty \left(\frac{(-1)^{n}}{(2n+1)!}\left(\frac{\pi}{2}\right)^{2n+1}\int x^{4n+2}\,dx\right)[/tex]​

To check s'(x), I used WolfrmAlpha.
 

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