\Psi(u,v)=\Psi(\lambda_1 e_1 + \lambda_2 e_2, \mu_1 e_1 + \mu_2 e_2) = \lambda_1 \Psi(e_1,\mu_1 e_1 + \mu_2 e_2) + \lambda_2 \Psi(e_2,\mu_1 e_1 + \mu_2 e_2)