The wave equation is obtained by letting Δ x → 0, in which case u i (t) takes the form u (x, t), where u (x, t) is continuous function of two variables, ü i takes the form ∂ u /∂ t, and u i + 1 + u i − − 1 − − 2 u i ( Δ Δ x ) 2 → → ∂ ∂ 2 u ∂ ∂ x 2 .