where the conserved quantity is y (r, t), ⋅ denotes the scalar product, ∇ is the nabla operator, here indicating a gradient, and a (y) is a vector of current coefficients, analogously corresponding to the divergence of a vector current density associated to the conserved quantity j (y) : y t + ∇ ∇ ⋅ ⋅ j ( y ) = 0