function f = rhsfun ( x )
%
%  f = rhsfun ( x )
%
%  RHSFUN returns the value of the right hand side function of the ODE.
%
%  X is the point of evaluation.
%
%  F is the value of the right hand side of the differential equation.
%
u = sin ( pi * x );
uxx = - pi * pi * u;
f = - uxx;