> Y1dot := diff(Y1, t) = -(diff(K, y1));

> Y2dot := diff(Y2, t) = -(diff(K, y2));

> y1dot := diff(y1, t) = diff(K, Y1);

> y2dot := diff(y2, t) = diff(K, Y2);


> DEplot([Y1dot, Y2dot, y1dot, y2dot], [Y1(t), Y2(t), y1(t), y2(t)], t = 0 .. 12, number = 4, {[y1(0) = 0, Y1(0) = 1, y2(0) = 0, Y2(0) = -4.17125]}, scene = [Y1, y1]);
%;
Error, (in DEtools/DEplot/CheckDE) derivatives must be given explicitly

what is the problem with this code? pleaaaaaaaaaaaaaaaaase help...

yes K is assigned something wlse and is my hamiltonian after a Lie transformation. y1 is the generalized coordinate in final target space and Y1 is the momentum of y1.

by writing two derivatives I wanted to write the hamilton's equations but I guess it is wrong. do you know how I should write the correct syntax?