In the general case a conservation equation can be also a system of this kind of equations (a vector equation) in the form: y t + A ( y ) ⋅ ⋅ ∇ ∇ y = 0 where y is called the conserved (vector) quantity, ∇ y is its gradient, 0 is the zero vector, and A (y) is called the Jacobian of the current density. In fact as in the former scalar case, also in the vector case A (y) usually corresponding to the Jacobian of a current density matrix J (y) : A ( y ) = J y ( y ) and the conservation equation can be put into the form: y t + ∇ ∇ ⋅ ⋅ J ( y ) = 0