thanks for your reply
Is there a way to also look at the third order derivative?
I tried diff(utt,t), and got zero, diff(diff(diff(u,t),t),t) did not work either
But diff(diff(diff(u^2,t),t),t) did work, why is that?
Eventually I want to put a third order derivative into the equation, and I wrote a term in the weak form to represent this as utt*test(ut), but it turned out it's not doing anything..
I saw your post in simwe.forum, sorry for late responding.
1) If you want to get thirder or higher derivatives of sloving variable u(x,t) with respect to time, a intermediate variable v(x,t)=diff(u(x,t),t) should be introduced. In other words, just keep the highest dirivative order of time that appears in any equation less than 3, otherwise you need to break it into sevral systems.
2) Acctually, diff(diff(diff(u(t)^2,t),t),t) =6*ut*utt+2*u*uttt.
Good Luck~
TBE