im trying to solve a PDE-System, with the "periodic boundary condition"
c[t,0]==4*c[t,2pi] But Mathenmatica says:
NDSolve::bcedge: "Boundary condition c[t,0]==4\ c[t,2\ [Pi]] is not specified on a single edge of the boundary of the computational domain."
If I replace the 4 by 1 everything is fine. Anyone can explain this to me? And/or can tell me how/if Mathematica can solve this?
The condition c[t, 0] == 4 * c[t, 2 Pi] does not define a periodic boundary condition. The function value must be the same for it to be periodic. That is also why it works if you replace 4 by 1. You might be able to replace c[t, x] by m[x] u[t, x] where u is periodic and m[x] grows exponentially by a factor of 4 over each period.
Here is an adaptation of the sine-Gordon equation in the NDSolve documentation illustrating this idea.
L = 4; m[x_] := Exp[Log[4] x/(2 L)]; sol = NDSolve[{D[m[x] u[t, x], t, t] == D[m[x] u[t, x], x, x] + Sin[m[x] u[t, x]], u[t, -L] == u[t, L], m[x] u[0, x] == Exp[-x^2], Derivative[1, 0][u][0, x] == 0}, u, {t, 0, L}, {x, -L, L}]; Plot3D[Evaluate[m[x] u[t, x] /. First[sol]], {x, -L, 5 L}, {t, 0, L}] 
A check of the constant ratio over the "period" 2 L:
Ratios /@ Table[m[x] u[t, x] /. First[sol], {t, RandomReal[{0, L}, 5]}, {x, -L, 7 L, 2 L}] (* {{4., 4., 4., 4.}, {4., 4., 4., 4.}, {4., 4., 4., 4.}, {4., 4., 4., 4.}, {4., 4., 4., 4.}} *)