This is an equation which describes the motion of an incompressible fluid and is given by 1. ∂∂t ui + j=13uj∂ui∂xj = ν Δ ui – ∂p∂xi + fi (x,t) x  R3, t ≥ 0, 1 ≤ i ≤ 3 2. div u = j=13∂uj∂xj = 0 x  R 3 t