function xxp = lotkavolterra(~,xx)
% LOTKAVOLTERRA Systeme predateur-proie Lotka-Volterra
%
%   Examples
%       sol  = ode23(@lotkavolterra,[0 20],[ 20 20 ]);
%       xint = linspace(0,20,200); 
%       yint = deval(sol,xint);
%       plot(xint,yint)
%       xlabel('temps'); ylabel('population'); 

x = xx(1); y = xx(2);
a = 1; b = 0.01;
c = 1; d = 0.02;

xxp = [ x * (a - b * y)
     -y * (c - d * x)];