function a = threed(n);
% THREED  a=threed(n) generates a 7-point matrix (3-D)
%   Uses the kronecker product.

% Edmond Chow

e = ones(n,1);
t = spdiags([e -2*e e], -1:1, n, n);
e = spdiags(e, 0, n, n);

a = kron(e,t) + kron(t,e);
e = ones(n,1);
e = spdiags(e, 0, n, n);
a = kron(e,a) + kron(a,e);

a=spones(a);
n=length(a);
a=-a+7*spdiags(ones(n,1),0,n,n);