Differential Equation w/ Power Series Solution

[DivGradCurl]

Problem

y^{\prime} = x^2 y

General Comments

There must be some kind of flaw in my solution as I don't get to the same result as the one my book provides:

y = c_0 \sum _{n=0} ^{\infty} \frac{x^{3n}}{3^n n!} = c_0 e^{x^3 / 3} \qquad \fbox{CORRECT ANSWER}

Any help is highly appreciated.

My Solution

If

y = \sum _{n=0} ^{\infty} c_n x^n \Longrightarrow x^2 y = \sum _{n=0} ^{\infty} c_n x^{n+2} = \sum _{n=3} ^{\infty} c_{n-3} x^{n-1}

and

y^{\prime} = \sum _{n=1} ^{\infty} n c_n x^{n-1}

Then

\sum _{n=1} ^{\infty} n c_n x^{n-1} = \sum _{n=3} ^{\infty} c_{n-3} x^{n-1} \Longrightarrow c_{n-3} = nc_n \quad n=3,4,5,\ldots

Hence, I ultimately get

y = c_0 + c_1 x + c_2 x^2 + c_0 \sum _{n=1} ^{\infty} \left[ \frac{x^{3n}}{3\cdot 6\cdot 9\cdot \cdots \cdot \left( 3n \right) } \right] + c_1 \sum _{n=2} ^{\infty} \left[ \frac{x^{3n-2}}{4\cdot 7\cdot 10\cdot \cdots \cdot \left( 3n-2 \right) } \right] + c_2 \sum _{n=2} ^{\infty} \left[ \frac{x^{3n-1}}{5\cdot 8\cdot 11\cdot \cdots \cdot \left( 3n-1 \right) } \right] \qquad \fbox{MY ANSWER}

Thank you very much!!

[Tide]

If you do your recursion relation carefully you will find that c_1 = c_2 = 0 since c_n = 0 for n < 0.

[DivGradCurl]

Oh... I've got it. Thanks

[dextercioby]

Oh... I've got it. Thanks

Cheating method:Why bother with stinky recursion relations,when u can use a series already known?? :wink:

\frac{dy}{dx} =y x^2 from which,by separating variables and integrating,one gets:
y=C\exp(\frac{x^3}{3}) .To find the series the problem asks you about,plug \frac{x^3}{3} in the power series of \exp{x} and you'll have your answer.

PS.This cheating trick works,as the differential eq.is easily solvable exactly.In general,it does not. :wink:

[DivGradCurl]

Just a final check...

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Method #1

How about...

y = c_0 + c_1 x + c_2 x^2 + c_0 \sum _{n=1} ^{\infty} \left[ \frac{x^{3n}}{3\cdot 6\cdot 9\cdot \cdots \cdot \left( 3n \right) } \right] + c_1 \sum _{n=2} ^{\infty} \left[ \frac{x^{3n-2}}{4\cdot 7\cdot 10\cdot \cdots \cdot \left( 3n-2 \right) } \right] + c_2 \sum _{n=2} ^{\infty} \left[ \frac{x^{3n-1}}{5\cdot 8\cdot 11\cdot \cdots \cdot \left( 3n-1 \right) } \right]

So, c_1 = c_2 = 0 ( I don't see clearly why ) . Then, we find

y = c_0 + c_0 \sum _{n=1} ^{\infty} \left[ \frac{x^{3n}}{3\cdot 6\cdot 9\cdot \cdots \cdot \left( 3n \right) } \right] = c_0 \sum _{n=0} ^{\infty} \frac{x^{3n}}{3^{n}n!} = c_0 e^{x^3 /3}

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Method #2

\frac{dy}{dx} = x^2 y \Longrightarrow \int \frac{1}{y}dy = \int x^2 dx

Then

\ln y = \frac{x^3}{3} \Longrightarrow y = Ce^{x^3/3}

where C = c_0