1. ## Sturm-Liouville eigenvalue problem

Hi,
I got a question which I'm quite stuck on...
Given the eigenvalue problem,
$y''+\lambda y=0$
$hy(0)+y'(0) = 0, y(1) = 0$
Show that if $\lambda = 0$ is an eigenvalue, then $h$ must have a specific value and find this value. If $h \geq 1$, show that there is exactly one negative eigenvalue.

I believe I've done the first part, as my working is as follows:
Rearrange the HLDE into self adjoint form: $-(-y')' = \lambda (-1)y$, and taking that $\lambda = 0$, means the HLDE becomes $y'' = 0$. The general solution to this should be of the form $y(x)= Ax + B$. Using the boundary conditions,
$hy(0)+y'(0) = hB + A = 0 \Rightarrow h = -\frac{A}{B}$
$y(1) = A + B = 0 \Rightarrow A = -B$
Therefore, $h = -\frac{(-B)}{B} = 1$
Does this look correct?

The next part of the question I get stuck on. The general solution for $-(-y')' = \lambda (-1)y$ is $y(x)= Ccos (\sqrt{\lambda}x) + Dsin(\sqrt{\lambda}x)$.
Using the boundary conditions
$hy(0)+y'(0) = h(Ccos (\sqrt{\lambda}0) + Dsin(\sqrt{\lambda}0)) + (-C\sqrt{\lambda}sin (\sqrt{\lambda}0) + D\sqrt{\lambda}cos(\sqrt{\lambda}0))$ $= h*C +\sqrt{\lambda}D = 0$
$y(1) =Ccos (\sqrt{\lambda}) + Dsin(\sqrt{\lambda}) = 0$
From here, I not sure where to go... have I do the right thing?

2. Originally Posted by Silverflow
Hi,
I got a question which I'm quite stuck on...
Given the eigenvalue problem,
$y''+\lambda y=0$
$hy(0)+y'(0) = 0, y(1) = 0$
Show that if $\lambda = 0$ is an eigenvalue, then $h$ must have a specific value and find this value. If $h \geq 1$, show that there is exactly one negative eigenvalue.

I believe I've done the first part, as my working is as follows:
Rearrange the HLDE into self adjoint form: $-(-y')' = \lambda (-1)y$, and taking that $\lambda = 0$, means the HLDE becomes $y'' = 0$. The general solution to this should be of the form $y(x)= Ax + B$. Using the boundary conditions,
$hy(0)+y'(0) = hB + A = 0 \Rightarrow h = -\frac{A}{B}$
$y(1) = A + B = 0 \Rightarrow A = -B$
Therefore, $h = -\frac{(-B)}{B} = 1$
Does this look correct? Yes.

The next part of the question I get stuck on. The general solution for $-(-y')' = \lambda (-1)y$ is $y(x)= Ccos (\sqrt{\lambda}x) + Dsin(\sqrt{\lambda}x)$.
Using the boundary conditions
$hy(0)+y'(0) = h(Ccos (\sqrt{\lambda}0) + Dsin(\sqrt{\lambda}0)) + (-C\sqrt{\lambda}sin (\sqrt{\lambda}0) + D\sqrt{\lambda}cos(\sqrt{\lambda}0))$ $= h*C +\sqrt{\lambda}D = 0$
$y(1) =Ccos (\sqrt{\lambda}) + Dsin(\sqrt{\lambda}) = 0$
You are looking for solutions with $\lambda <0$, so it's not a good idea to use functions like $\cos(\sqrt{\lambda}x)$ and $\sin(\sqrt{\lambda}x)$. Instead, notice that the equation $y'' = (-\lambda) y$ has solutions $y=e^{\pm\alpha x}$, where $\alpha = \sqrt{-\lambda}$. If $y = Ae^{\alpha x} + Be^{-\alpha x}$ then the boundary conditions lead to some equation like $h = \frac{\alpha(e^{\alpha} + e^{-\alpha})}{e^{\alpha} - e^{-\alpha}}$, and you need to show that for $h\geqslant1$ that has a unique solution for $\alpha>0$.

3. That makes a lot more sense. Thank you very much!