predprey2.m

来自「ode routines in matlab」· M 代码 · 共 35 行

function [out] = PredPrey2(x, y,flag)
%
%  Evaluate the right hand side function for the
%  Predator Prey model
%
%                                         y_1(x) * y_2(x)
%    y_1'(x) = 1.2 * y_1(x) - y_1^2(x) -  ---------------
%                                          y_1(x) + 0.2
%
%               1.5 * y_1(x) * y_2(x)
%    y_2'(x) =  ---------------------  -  y_2(x)
%                   y_1(x) + 0.2
%
%
%



switch( flag )
   case '' % return right hand side

      out  = [ 1.2*y(1) - y(1)^2 - (y(1)*y(2)) / (y(1)+0.2);
               (1.5*y(1)*y(2)) / (y(1)+0.2) - y(2) ];
   
   case 'jacobian' % return Jaacobian of right hand side

      out  = [ 1.2 - 2*y(1) - y(2)/(y(1)+0.2)^2   y(1) / (y(1)+0.2) ;
               0.3*y(1)/(y(1)+0.2)^2              (1.5*y(1)) / (y(1)+0.2) - 1 ];
   
   otherwise
      error(['Unknown request.'])
end