function yprime = vdpol (y,t,mu);
%
%  yprime = vdpol (y,t,mu);
%
%  Evaluates the derivative f(y,t) of the van der Pol eqn.
%  The parameter mu determines the strength of the nonlinearity
%  (mu=0 is the harmonic oscillator).
%

%%  Note:  In Matlab, everything following a percent sign(%) is a
%%  comment.  All comments prior to the first blank line
%%  of a file are shown with the command 'help vdpol'
%%  and typically contain usage information.

%%  A matrix is denoted by square brackets.
%%  The elements are given row-wise, with columns 
%%  separated by spaces or commas, and rows separated by semicolons
%%
%%  Examples:  
%%  a = [ 1, 2] is a row-vector,
%%  b = [ 1; 3] is a column-vector
%%  c = [ 1,  2; 3, 4] is a matrix
%%

   yprime = [y(2); mu*(1-y(1)^2)*y(2) - y(1)];

