function xp=F(t,x);
% F right hand side for simplistic crude oil forcasting model ODE
%   xp=F(t,x); right hand side function of the ODE x'=F(t,x,a), for a
%   simplistic crude oil forcasting model depending on the parameters
%   given in the global vector a

global a;
  
xp(1,1)=-x(2);
xp(2,1)=sign(a(1)*x(1)-a(2)-a(3)^2*x(2))*...
        abs(a(1)*x(1)-a(2)-a(3)^2*x(2))^a(4)*x(2);