function A=helmA(N,w)
    ee=ones(N,1);
    h = 1/(N+1);
    %a=4; b=-1-gamma; c=-1-beta; d=-1+beta; e=-1+gamma;
    t1=spdiags([-ee,4*ee,-ee],-1:1,N,N);
    t2=spdiags([-ee,zeros(N,1),-ee],-1:1,N,N);
    A=kron(speye(N),t1)+kron(t2,speye(N)) - w^2*h^2 *speye(N^2);
    