function ydot = f(t, y)
% F :  function to be "integrated" to solve an ODE
%
% The differential equation is ydot = f(t, y)
%
%        t: time, a scalar.
%        y: unknown function of time: y = y(t). Can be a vector.
%
%        ydot: time derivative of y: dy/dt. Vector the same size as y.
%

ydot = y/2;