%
%  function for the definition of the ODE system
%
%  y1'=y2
%  y2'=-y1
%  y1(0)=1
%  y2(0)=0
%
function res = f1( x, Y )
  res = [ Y(2) ; -Y(1) ] ; % return a matrix with 2 row and 1 column 
end