assume(omega > 0)$ ode2( 'diff(y, t, 2) + omega^2* y= 0, y, t ); ic2(%, t = 0, y = a, 'diff(y,t) = 0); y=a*cos(omega*t) is the answer ... I am trying to get a sin ... instead a cos but I can't !!! I am not an expert in maths, but as this question arise in my mind, I would like to know an answer ? Any help welcome Kind regards, Henri