I see. Indeed i used Ctrl + ^ instead of Ctrl + % for the upper bound in the integral, which i changed now.

Now it says " Input is not an ODE ", which is probably cause by the integral. From a physical point of view i dont think i can just differentiate to get rid of it.

But i'll think about that. Thanks so long.

Edit: But that error even stays, when i replace the integral by 1, so probably it isnt just because of the Integral.

Edit 2: Ok, i think i got that too. It seems like even though n is only a function of t, one has to declare it as a function of t and theta to get mathematica solving it. It even accept the integral. Only issue that is unsolved is my boundary condition. Mathematica says:

"Boundary condition c[t,0]==4.\ c[t,2\ \] is not specified on a single edge of the boundary of the computational domain."

Which i dont understand, because it is specified on both edges of the boundary like a periodic boundary condition.