function f = laplacian(x, sigma)
% Laplacian
% INPUT: vectors 'x' and parameters
% OUTPUT: vector 'f'
%  = sigma^2 G_{xx}
%   (G is gaussian, G_{xx} is second derivative)
% Note: the function is scale-invariant

f = -1/sqrt(2*pi)/sigma*exp(-x.^2./2./sigma^2).*(1-x.^2./sigma^2);