function w0 = wzero(x,k,lx,mu)
% WZERO defines initial condition  u0 = u(x,0), u0_t = u_t(x,0)

amp0 = 2.0;
fracshift = 1/pi;
lshift = round(lx*fracshift);

u0 = uzero(x,lx);
% u0_t =  -sqrt(mu)*amp0*deriv(u0,k);
u0_t = zeros(1,lx);

u0 = circshift(u0,lshift);
u0_t = circshift(u0_t,lshift);

w0 = [u0,u0_t].';