function d=vanderPol(t,y)
% vanderPol(t,Y) evaluates the right side of the Van der Pol equation,
%                     y''+a*(y^2-1)*y'+y=0,
% after conversion to a system in the standard way: Y=[y,y'].  
% a=parameter (global).

global parameter
d=[y(2);parameter*(1-y(1)^2)*y(2)-y(1)];
% M. Day, June 4, 2005