- #1
brkomir
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Homework Statement
We have a linear differential operator ##Ly=-y^{''}## working on all ##y## that can be derived at least twice on ##[-\pi ,\pi ]## and also note that ##y(-\pi )=y(\pi )## and ##y^{'}(-\pi )=y^{'}(\pi )##.
a) Is ##0## eigenvalue for ##L##?
b) Is ##L## symmetric? (I think the right English expression would be self-adjoint)
c) Find all positive eigenvalues and corresponding eigenfunctions. What is the dimension of eigenspace?
d) Does ##L## have any negative eigenvalues?
Homework Equations
The Attempt at a Solution
a) No idea how to do this one. Here's my version:
##Ly=-y^{''}=\lambda y## which gives me ##y^{''}+\lambda y=0##. Solution of this differential equation is obviously ##y(x)=Acos(\sqrt{\lambda }x)+Bsin(\sqrt{\lambda }x)##
Using this we find out that the boundary conditions for ##A\neq 0 ## and ##B\neq 0## are fulfilled only when ##\lambda =0##. Therefore the answer to question a) is YES.
b)
##L## is symmetric if ##[PW(y,x)]\mid _a^b =0##. Note that originally the DE is ##P(z)y^{''}+Q(z)y^{'}+R(z)y=0## and where ##W## is Wronskian determinant.
Obviously ##P=-1##.
##[PW(y,x)]\mid _a^b =[-\begin{vmatrix}
y & z\\
y^{'} & z^{'}
\end{vmatrix}]\mid _a^b##
After short calculus and knowing that ##a=-\pi ## and ##b=\pi ## we find out that the equation above is equal to ##0## and therefore the linear operator ##L## is symmetric.
c)
##Ly=-y^{''}=\lambda^2 y## where I used notation ##\lambda ^2## for eigenvalue instead of ##\lambda ## just because I don't want to write square roots every time.
Anyhow, solution to this DE is ##y(x)=Acos(\lambda x)+Bsin(\lambda x)## using boundary conditions:
##y(\pi )-y(-\pi )=2Bsin(\lambda \pi )=0## and of course for non trivial solutions ##B\neq 0## than:
##\lambda =n## for ##n\in \mathbb{Z}^{+}## where i suspect these are all the positive eigenvalues.
And accordingly, I assume that ##y(x)=Acos(\lambda x)+Bsin(\lambda x)## are eigenfunctions where for each ##\lambda ## the dimension of the eigenspace increases by ##1##, therefore the dimension of eigenspace is ##n##.
(or do I have to consider ##\lambda =0## here too?)
d) Hmmm, if ##\lambda <0 ## than the differential equation ##y^{''}+\lambda y=0## changes to ##y^{''}-\lambda y=0##.
Now ##y## that solves the equation above is ##y(x)=Ae^{\sqrt{\lambda }x}+Be^{-\sqrt{\lambda }x}##.
Again, taking in mind that ##y(-\pi )=y(\pi )## leaves me with
##(B-A)(e^{-\sqrt{\lambda }x\pi }-e^{\sqrt{\lambda }x\pi })=0## which is only true if ##(B-A)=0##.
Therefore the answer is NO, ##L## does not have negative eigenvalues.