Re: Solving ODE

[Carlo de Falco]

Hi,

On 08/mag/08, at 01:08, fraud profile wrote:

> Hi,
>
> I am using lsode to solve a simple ODE on Octave 2.1.72.
> The ode is:
> Solve: y''(t) + y(t) =0
> such that:
> y(0)=3 & y(pi/2)=3

LSODE is intended for solving intial value problems,
what you have is a boundary value problem.

if you install the BIM package from octave-forge you can solve your
equation by:

Nnodes = 100;
Nelements = Nnodes-1;
x = linspace(0, 1, Nnodes)';
y(Nnodes, 1) = y_pihalf = 3;
y(1, 1) = y_0 = 3;

A =  BIM1Alaplacian(x, -ones(Nelements, 1));
A += BIM1Areaction(x, ones(Nelements, 1), -ones(Nnodes, 1));

y(2:end-1) = A(2:end-1,2:end-1) \ ( - A(2:end-1, [1 end]) * [y_0;  
y_pihalf]);
plot(x,y)

c.