function l = ptlaplacian(u,g,k,m)

v = address(k,m) ;
vnbhd = vneighbors(v,m) ;
gnbhd = gneighbors(v,m) ;
alpha = log(3)/log(5/3) ;

l = 0 ; 
for j = 1:4
  gxy = g(indexsg(gnbhd(j,:),m)) ;
  l = l + gxy^-(1+alpha)*(u(k)-u(indexsg(vnbhd(j,:),m))) ;
end
