function C = condif(n,c)
% C is a square convection-diffusion matrix 
% of order n^2.  The user must supply n and the
% convection coeffients c(1:2)
I = speye(n);
N = sparse(1:n-1,2:n,1,n,n); 
F1 = 2*I - (1-c(1))*N - (1+c(1))*N';
F2 = 2*I - (1-c(2))*N - (1+c(1))*N';
C = kron(I,F1) + kron(F2,I);