%
% function for the IVP
%
%  x'(t) = -y(t)
%  y'(t) = x(t)
%
%  x(0) = 1
%  y(0) = 0
%
%       / x \
%  Z = |    |
%       \ y /
%
function f = problem1( t, Z )
  f    = zeros(2,1) ; % allocate for a 2 elements column vector
  f(1) = -Z(2) ;
  f(2) =  Z(1) ;
end