I am writing a program to obtain the solution of an undamped free vibration system. At first, I used the Euler method and the resulting time-domain curve seemed normal (perfectly sinusoidal with the initial displacement as the amplitude). But when I checked the peaks using findpeaks, its value increased continuously (0.1, 0.1001, 0.1002, and so on). Then, I used the ode45 to do the same. The peaks of the time-domain curve decreased continuously from the initial displacement (0.1, 0.0999, 0.0998, and so on). Why is this happening? Am I doing something wrong?