function dw = sgrhs(t,w,flag,D)%  The function dw = sgrhs(t,w,flag,D) computes the right-hand side%  of the sine-Gordon system with the aid of an NxN differentiation matrix D.%  The input parameter flag is set by the ODE solver.%  J.A.C. Weideman, S.C. Reddy 1998 N = length(w)/2; u = w(1:N); v = w(N+1:2*N);du = v;dv = D*u-sin(u);dw = [du; dv];