function K =commute(m,n)
Im=speye(m);
In=speye(n);
K=sparse(m*n,m*n);
if m <= n;
  for i=1:m;
    K=K+kron(kron(Im(:,i)',In),Im(:,i));
  end;
else
  for i=1:n;
    K=K+kron(kron(In(:,i),Im),In(:,i)');
  end;
end;