For the inhomogeneous system (2) we must solve ⃗ x = A⃗ x + ⃗ f (t) subject to ⃗ x 0 ≡ ⃗ x(t 0 ) where ⃗ f (t) is a column vector function of the same dimension as ⃗ x(t) and A now represents the associated 2 × 2 matrix. The unique solution is