function dy = f(x,y);

% I want to compute the ODE dy/dx = f(x,y) = cos(x)*y
dy = cos(x)*y;

% For the pendulum, I have theta'' + sin(theta) = 0.    
%
%     theta' = v
%       v'   = - sin(theta)
%
% where theta = y(1) and v = y(2). I need dy to be a column vector.
%
% dy(1,:) = y(2);
% dy(2,:) = -sin(y(1));