%
% exact solution for the IVP
%
%  x'(t) = -y(t)
%  y'(t) = x(t)
%
%  x(0) = 1
%  y(0) = 0
%
%       / x \
%  Z = |    |
%       \ y /
%
%  x(t) = cos(t)
%  y(t) = sin(t)
%
function Z = exact1( t )
  Z      = zeros(2,size(t,1)) ; % allocate for a 2 elements column vector
  Z(1,:) = cos(t) ;
  Z(2,:) = sin(t) ;
end