1
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0answers
22 views

Power series solution for a DE with Frobenius method

The given DE is $(x²-3)y"+2xy'=0$ Since there is a singular point ($x=\pm\sqrt{3}$) I used the Frobenius method. I found two indicial relationships: $-3r(r+1)=0$ and $-3(r+1)(r+2)=0$ because I have ...
2
votes
0answers
38 views

Power series to solve differential equations?

We can use the formula $$F(x)=e^{λx} [ ρ-λμ-\dfrac{1}{2} λ^2 σ^2 ]^{-1}. (1) $$ to derive an expression for F(x) when f(x) is any integer power $x^n$. Begin by observing that for the ...
0
votes
1answer
14 views

Why not shift the index of the derivative in Euler series?

I'm reading over solving linear differential equations with analytic coefficients, and finding the solutions that are near regular singular points. In the earlier section on solving similar equations ...
0
votes
0answers
27 views

Radius of Convergence and the Frobenius Method

Consider the equation $$4xy'' + 2y'+ y = 0$$ I know that $x=4$ is a regular singular point, and in the notation that my uni uses, we say that: $$(x-x_0)^2 y'' + (x-x_0)p(x)y' + q(x)y = 0$$ where ...
0
votes
1answer
15 views

Limit when $y>>a$ of a derived solution

I am able to do part d), however I am very stuck on part e). If $y >> a$ then surely we get $\phi(x,y)$ $= \frac{1}{\pi} \Big[ tan^{-1} \Big( \frac{x+a}{y}\Big)-tan^{-1} ...
1
vote
0answers
43 views

A theoretical question regarding Frobenius method

The following is a theoretical question regarding Frobenius method. Let $b(x),c(x)$ be real functions analytic at 0. Let $b(x)=\sum_{i=0}^\infty b_ix^i, c(x)=\sum_{i=0}^\infty c_ix^i$ on $(-R,R)$. ...
1
vote
1answer
41 views

Power Series Solution to Differential Equation

The equation is $$y'' - xy' + y = 0$$ So far I have the recurrence relation - $$a_{n+2} = \dfrac{(n-1)a_n}{(n+1)(n+2)} $$ From this - $a_2 = \dfrac{-a_0}{2!}$ $a_3 = 0$ $a_4 = ...
0
votes
1answer
20 views

Series Solution To Differential Equations - Need help with one step

Would someone kindly explain to me what the logic is behind one of the steps here: http://tutorial.math.lamar.edu/Classes/DE/SeriesSolutions.aspx In Example 1 - Following on from this sentence on ...
2
votes
1answer
24 views

Power series solutions of differential equations, choosing x^n or x^(n+r)?

I cannot understand which one to use when solving differential equations by using power series solutions. For example in this question: Consider the following differential equation for $\alpha \in ...
0
votes
1answer
40 views

Differential equation by series solution method: equating coefficients to zero

I am following the solution for a problem, and I am stuck at the following equation: $$2a_2+\sum_{n=1}^\infty \left[(n+2)(n+1)a_{n+2}-a_{n-1}\right]x^n=0\tag1$$ Now, the professor equates the ...
0
votes
1answer
28 views

What is the significance of finding the series solution of a differential equation “about a point”?

I am learning the series solution method of solving differential equations, and I am curious as to what the rationale is for finding out the solution of the equation about a particular point. It seems ...
2
votes
0answers
29 views

Solution regarding Power Series and ODE's

About 4 months ago I posted Series solution to $y''-xy'-y=0$. I ran through the analysis and it appeared that I solved the ODE . The solution seemed to be ...
0
votes
0answers
51 views

How to determine undetermined coefficients of infinite series solution of ODE by seeking the solutions that vanish at infinity?

For an ODE like $f''+p(x)f'+q(x)f=r(x)$ where $p,q,r$ are analytic functions I'm trying an infinite series solution $f=\sum_{n=0}^\infty a_n x^n$. All coefficients $a_n$ can be obtained in terms of ...
0
votes
1answer
123 views

If $f(2x)=2xf'(x)$, then find $f(x)$

If $f(x)$ is Analytic functions on $R$,and such $$2xf'(x)=f(2x)$$ Find all $f(x)$ My idea: let $$f(x)=\sum_{n=0}^{\infty}a_{n}x^n$$ so I can't Thank you
1
vote
1answer
85 views

Solving differential equations using power series

I need to solve this differential equation by power series: $$y''+3xy'+(2x^{2}+6)y=0$$ Any help is great! Thanks!
0
votes
1answer
47 views

please solve this diffrential equation question on power series

In the differential equation $y'' + (x-3)y' + y=0 $ of power series at $x_0=2$ , I took $ y=\sum_{n=0}^{\infty}a_n(x-x_0)^n $ ,then I tried to solve this but not getting the answer. if someone solve ...
0
votes
3answers
49 views

Problem about ODE and power series

For each $a \in \mathbb{Z}^+$ let the following ODE $$ x'' - \dfrac{a (a+1)}{(1 +t^2)} x = 0$$ Using power series around the origin, show that the equation has a solution $p_a(t)$ which is a ...
1
vote
1answer
54 views

Solve ODE using analytic solutions

Let the following ODE: $x'' + tx' + x = 0.$ Find the general solution $x(t) = a_0 x_1(t) + a_1 x_2(t),$ with $a_0, a_1 \in \mathbb{R}$ and $x_1(t), x_2(t)$ are $t$ power series convergent for ...
1
vote
0answers
74 views

Find the indicial equation of $(x+2)^2(x-1)y''+5(x-1)y'-\pi(x+2)y = 0$

Find all singular points of each equation, and determine whether they are regular or irregular. At each regular singular point, find the indicial equation and the exponents of singularity. ...
2
votes
3answers
126 views

Find the form of a second linear independent solution when the two roots of indicial equation are different by a integer

Consider the differential equation $$x^2y''+3(x-x^2)y'-3y=0$$ $(a)$ Find the recurrence equation and first three nonzero terms of the series solution in powers of $$ corresponding to the larger root ...
2
votes
1answer
72 views

Find one series solution for $xy'' - y = 0$

I have found the recurrence relation to be $a_{n+1} = \frac{(a_n)}{(n+1)(n)}$ . I am stuck at this part because no matter what I set the initial a to be, the following term will have a problem due to ...
0
votes
2answers
78 views

Solving differential equation by using power series.

Find, using the power series: $$y(x)=\sum_{k=0}^\infty a_{k}x^k$$ a solution for the following differential equation: $$y'(x) = -x^2y(x),\,\, y(0)=1$$ What's the convergence radius of the constructed ...
2
votes
1answer
56 views

Why don't power series methods work for linear ODE's with singularities?

My math class tells me power series methods don't work for equations of the form $$f'' +p(x)f' +q(x)f = 0$$ if the functions $p(x)$ or $q(x)$ have singularities at the point about which you're ...
1
vote
1answer
98 views

Prove $\sum_{m \geq 1} {\frac{(2m-2)!}{(1-\rho)\cdots(m-\rho)} \frac{t^m}{(1-x)^{2m-1}}} $is divergent

How do I show that the following power series is divergent? $$ u(t,x) = \sum_{m \geq 1} {\dfrac{(2m-2)!}{(1-\rho)\cdots(m-\rho)} \dfrac{t^m}{(1-x)^{2m-1}}} $$ where $t$ is complex 1-dimensional, $x$ ...
0
votes
2answers
46 views

a differential equation equation related to fourier series

I am really struggling with this one. Any help is welcome! For equation $f''(z) + p(z) f'(z) + q(z) f(z) = 0$, where $p(z)$ and $q(z)$ are fixed polynomials. Given $f(0)=f_0$, $f'(0)=f_1$, prove that ...
1
vote
2answers
62 views

Series Solution of Second Order Linear Equation, IVP

Consider the initial value problem $$y' = \sqrt{1-y^2}$$ $$y(0) = 0$$ Look for a solution of the IVP in the form of power series about x=0. I have started with assuming that $ y = \sum_{n = ...
0
votes
1answer
150 views

Finding a solution in powers of $(x^2-1)y''+4xy'+2y=0$

I'm working on finding the general solution of $(x^2-1)y''+4xy'+2y=0$ in powers. I assume the form: $$ y(x)=\sum_{n=0}^\infty C_nx^n$$ My basic strategy is to first figure out each piece ...
1
vote
1answer
83 views

problem with recurrence relation for series solution for ODE

I have $$y''-xy'-y=0$$ and I'm trying to find the series solution around the ordinary point $x_0=1$. My last post I muscled through to the solution when the ordinary point was $x_0=0$, but this is ...
2
votes
1answer
88 views

Choice of the First Term in Legendre Polynomials

The two solutions of the Legendre's Differential Equation obtained by series solution method are : and Now according to my textbook, for the useful polynomial for n equal to a positive integer, ...
5
votes
2answers
102 views

Values of $k$ for non-trivial solutions of the differential equation $y''-\left(\frac{1}{4}+\frac{k}{x}\right)y=0$ where $x$ is non-negative

I attempted a power series solution of this equation in order to find the values of k that have a non-trivial solution: $y''-\left(\dfrac{1}{4}+\dfrac{k}{x}\right)y=0$ I am having trouble ...
1
vote
3answers
130 views

Find a power series solution centered at 0 (Differential equations

Here's the problem: $$(x-1)y''+y'=0$$ This is the work that I've already done: $$y=\sum_{n=0}^{\infty}a_{n}x^n$$ $$y'=\sum_{n=0}^{\infty}(a_{n+1})(n+1)x^n$$ ...
2
votes
0answers
36 views

Power Series for Original Differential Equation

The question: $y"+x^2y'+2xy=0$ I continue to get the incorrect answer and not sure why. I changed my indices around to make x^n all throughout and that's where the trouble starts. My answer ...
1
vote
0answers
142 views

Second Series solution y(2) for Frobenius Method

I am currently solving the Frobenius Method for the question $xy'' +y = 0$ given the ICs $y(0) = 0, y'(0)=1$ I have done some work into solving that the first series solution for $y_1 = ...
2
votes
1answer
61 views

power series method to solve ODE

Using power series method, solve Airy’s equation $$y′′+ xy = 0$$. How do I start solving this? Thanks in advance!
0
votes
1answer
119 views

Differential Equations: Find the first four terms in each of two solutions y1 and y2 …

The differential equation is $y'' - xy' - y = 0$ with $x_0 = 1$ Now, I know how to find the recurrence relation... and it's given by: $a_(n+2) = [(a_(n+1) + a_(n)) / (n+2)]$ But I can't quite ...
0
votes
0answers
27 views

Compute coefficients of a rational expansion

I am approximating certain solution of an ODE by power expansions. As it is customary, I propose an ansatz and then I check for the coefficients to satisfy the ODE. At some point of my computations I ...
4
votes
1answer
162 views

Simple Frobenius problem without recurrence relation?

I am just learning frobenius method in my 'math methods in physics' class. The first problem i am trying to solve is $$ x^2y''-xy'+n^2y=0$$ (where n is a constant). I know that i have to plug in the ...
0
votes
0answers
29 views

Extending a power series?

I am studying a differential equation $$ y'(x)=g(x,y), $$ which has no analytic solution, however I have found that $y(x)$ is asymptotic to a series $$ f(x)=\sum_{k=0}^\infty a_kx^{-k} $$ as ...
3
votes
1answer
237 views

How to identify this power series as $k\sin(k/x)$?

In this question, a functional equation is solved for functions with a power series. We find a recursive formula: (copied from the answer by user achille hui) \begin{align} ( 2^1 - 3 ) a_2 &= 0\\ ...
0
votes
1answer
113 views

Solve $2(x+1)y' = y$ using Power Series.

Given the ODE: $2(x+1)y' = y$ How can I solve that using Power Series? I started to think about it: $ \\2(x+1)\sum_{n=1}^{\infty}{nc_nx^{n-1}}-\sum_{n=0}^{\infty}{c_nx^n}=0 ...
0
votes
1answer
118 views

If $f(x,y,t):= u(r) \cos ( \omega t)$, use the multivariable chain rule to obtain an ODE for $u$ from the PDE for $f$.

Let $f(x,y,t) :=u(r)\cos \omega t$, where $r= \sqrt{x^2 +y^2}$. Physics tells us the following: For $f(x,y,t)$ to describe a vibrating membrane, with $f(x,y,t)$ telling how high the mem- brane is ...
0
votes
2answers
158 views

Finding Coefficients of Power Series Expansion

Find the coefficients of $ a_{n}$ and $b_{n}$ for $ 0 ≤ n ≤ 4$ for the power series expansion of two linearly independent solutions of the ODE: $y'' -(e^{x}-1)y=0$. This is what I've tried so far: ...
1
vote
0answers
60 views

Solving ODE with negative expansion power series [duplicate]

I am solving a series of ODE, such that each DE is equal to some degree of term that I'm expanding to. For instance, one DE is this: $\xi^r\partial_r g_{rr}+2g_{tt}\partial_t\xi^t=\mathcal{O}(r)$ ...
0
votes
1answer
53 views

Simple differentiation question that I am unsure about

I am in the process of re-learning differentiation and am stuck on this as part of a larger problem. Can you explain to me why when differentiated 4 times this: $$y = \sum_{n=0}^{+\infty} ...
1
vote
1answer
121 views

Finding coefficients of a differential equation represented by power series

I am studying for a discrete mathematics exam and have gotten stuck on this question: Any function y of a real variable x that solves the diff erential equation: $$\frac{d^4y}{dx^4} -16y =0$$ may ...
0
votes
1answer
149 views

Why use series solution rather than variation of parameters?

When should we use series solution to solve a general 2nd order ODE rather than the variation of parameters? Could both methods be used to solve any 2nd order ODE or are there restrictions on when ...
2
votes
3answers
161 views

How maths can help to compute convergence?

$r'(\theta)^2 + r(\theta)^2 = \theta^2,\quad r(t=0)=0\tag{1}$ There is an interesting approach to prove that the solutions of the equation $(1)$ have power series representations of form ...
1
vote
0answers
86 views

$\sum_{k=0}^{\infty}\frac1{4^k(2k+1)}\binom{2k}{k}x^{2k+1}\sum_{k=0}^{\infty}(-1)^k\frac1{4^k}\binom{2k}{k}x^k =$

Prove that for $|x|<1$, $\sum_{k=0}^{\infty}\frac1{4^k(2k+1)}\binom{2k}{k}x^{2k+1}\sum_{k=0}^{\infty}(-1)^k\frac1{4^k}\binom{2k}{k}(-x^2)^k = ...
5
votes
1answer
426 views

When solving an ODE using power series method, Why do we need to expand the solution around the singular point?

When solving a differential equation using series expansion method, if it has the following form : $$y''+\frac{p(x)}{x}y'+\frac{q(x)}{x^2}y=0$$ ; where $p$ and $q$ are analytic at $x_0$; if we want ...
0
votes
1answer
368 views

Method for solving ODE with power series

when trying to solve second order linear homogeneous variable coefficient ODEs using a power series method, there seem to be two different general forms cropping up in my notes. The first uses an ...