       How to find the eigenvalues/eigenfunctions of a differential linear

• To: mathgroup at smc.vnet.net
• Subject: [mg113380] How to find the eigenvalues/eigenfunctions of a differential linear
• From: "Nasser M. Abbasi" <nma at 12000.org>
• Date: Tue, 26 Oct 2010 05:34:06 -0400 (EDT)

```This is a simple math problem, which I solved by hand, but having hard
time to get the same result I had by using Mathematica.  I'll describe
the problem, the solution I obtained by hand, what I did to try to
verify my result.  I'd like to be able to get Mathematica to produce the
same result I have.

Summary: I need to have mathematica find the eigenvalues and
eigenfunctions of differential operator without me having to do too much
manual manipulation.

Problem
--------
Given Lu==u''[x], with boundary conditions u'==0,u'==0, need to
find the eigenvalues and eigenfunctions of L.

my solution:
------------
The eigenvalues are

lambda[n_]= -(n Pi)^2
for n=0,1,2,3,... infinity

with corresponding eigenfunctions

Phi[n_,x_]:=Cos[ n Pi x]
for n=0,1,2,3,... infinity

So, the first eigenvalues are

In:= lambda[n_]:=-(n Pi)^2
Table[lambda[i],{i,0,5}]//N
Out=
{0.,-9.869604401089358,-39.47841760435743,-88.82643960980423,-157.91367041742973,-246.74011002723395}

What I tried in Mathematica
----------------------------

Tried to solve u''[x] == lambda * u[x], i.e. eigenvalue problem with the
BVP above, but I get zero solution

DSolve[{ u''[x]- lambda u[x]==0,u'==0,u'==0},u[x],x]

{{u[x]->0}}

Now tried just one BC, I picked one of the above 2, but I also tried the
other one later, no success, do not get what I had.

Clear[u]
First@DSolve[{ u''[x]- lambda u[x]==0,u'==0},u[x],x];
ExpToTrig[%]//Simplify

{u[x]->2 C Cosh[Sqrt[lambda] x]}

u=u[x]/.%

2 C Cosh[Sqrt[lambda] x]

% find root of the above eigenfunction

FindRoot[(u/.{C->1,x->1})==0,{lambda,1}]

{lambda->-2.46740110027234}

% try to guess what the form is

Sqrt[-lambda/.%]

1.5707963267948968

% see what it is

Pi/2.
1.5707963267948966

Ok, so  Sqrt[-lambda] = pi/2, then the (first?) eigenvalue is -(pi/2)^2
The rest, keep looking for next roots of the eigenfunction from above?

But I get eigenvalue to be   -(n*pi)^2, not  -(n* pi/2)^2 anyway. Might
have made a mistake there at the end.

The bottom line is this:  Is there a package or command to make it
easier to find the eigenvalues and eigenfunctions of a linear operator
in mathematica, without having to hunt for them by hand? One has to use
one B.C. to get something, and them hunt for roots, and guess the form.

It seems this task can be automated by some command or package if not