function [ k ] = ker(x,y,u,v,xi1,xi2,a)
% kernel of the NFE acording to the example of Faye Faugeras (4)
%  
% 
k=1./sqrt(2*pi*xi1^2)*exp((-(x-u)^2-(y-v)^2)/(2*xi1^2))-...
   a/sqrt(2*pi*xi2^2)*exp((-(x-u)^2-(y-v)^2)/(2*xi2^2));
end

