Hi, I am a new Fenics user, I just confused with set up the weak form for one equation, I appreciate it if someone can help me with this.

So I already derived the equation by FDM, just not sure how to write the weak form in fenics. The equation:

u(z,t)^(n+1)-u^(n)-4*a*delta_t*C*pi^2*(u(z,t)^(n+1))^3*sec^2(pi*(u(z,t)^(n+1)-B)/(A-B))*(d/dz(u(z,t)^(n+1)))^2=0, where a,A,B,C are constant, u^(n) is u at time level n.

Many thanks