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apu's question at Yahoo! Answers regarding the number of positive roots of solution to ODE

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MarkFL

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Feb 24, 2012
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Here is the question:

Show that this differential equation has infinite positive zeros?


Show that every nontrivial solution for y''+ (k/x^2) y=0, has an infinite number of positive zeroes if K>1/4 and only a finite number if K<1/4 and if K=1/4.
I have posted a link there to this thread so the OP can see my work.
 
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MarkFL

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Feb 24, 2012
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Re: apu's question at Yahoo! Anwers regarding the number of positive roots of solution to ODE

Hello apu,

We are given the 2nd order linear ODE:

\(\displaystyle y''+\frac{k}{x^2}y=0\)

Multiplying through by $x^2$, we obtain the Cauchy-Euler equation:

\(\displaystyle x^2y''+ky=0\)

Using the substitution $x=e^t$, we find:

\(\displaystyle x^2\frac{d^2y}{dx^2}=\frac{d^2y}{dx^2}-\frac{dy}{dt}\)

Hence, the ODE is transformed into the linear homogenous ODE:

\(\displaystyle \frac{d^2y}{dx^2}-\frac{dy}{dt}+ky=0\)

The characteristic roots are:

\(\displaystyle r=\frac{1\pm\sqrt{1-4k}}{2}\)

We know the nature of the solution depends on the discriminant.

Case 1: The discriminant is positive.

\(\displaystyle 1-4k>0\)

\(\displaystyle k<\frac{1}{4}\)

The solution is then:

\(\displaystyle y(t)=c_1e^{\frac{1+\sqrt{1-4k}}{2}t}+c_1e^{\frac{1-\sqrt{1-4k}}{2}t}=e^{\frac{1-\sqrt{1-4k}}{2}t}\left(c_1e^{\sqrt{1-4k}t}+c_2 \right)\)

We conclude that the solution has no positive roots.

Case 2: The discriminant is zero.

\(\displaystyle 1-4k=0\)

\(\displaystyle k=\frac{1}{4}\)

In this case, because of the repeated characteristic root, the general solution is:

\(\displaystyle y(t)=c_1e^{\frac{t}{2}}+c_2te^{\frac{t}{2}}=e^{ \frac{t}{2}}\left(c_1+c_2t \right)\)

Here, we find that there can at most one positive real root.

Case 3: The discriminant is negative.

\(\displaystyle 1-4k<0\)

\(\displaystyle \frac{1}{4}<k\)

The general solution is then given by:

\(\displaystyle y(t)=e^{\frac{t}{2}}\left(c_1\cos\left(\frac{\sqrt{1-4k}}{2}t \right)+c_2\sin\left(\frac{\sqrt{1-4k}}{2}t \right) \right)\)

The sinusoidal factor guarantees an infinite number of positive real roots.