The dynamical behaviour of physical processes is usually modelled via differential equations. But if the states of the physical system are in some ways constrained, like for example by conservation laws as Kirchhoff's laws in electrical networks, or by position constraints as the movement of mass points on a surface, then the mathematical mode) also contains algebraic equations to describe these constraints. Such systems, containing both differential and algebraic equations are called differential-algebraic systems, algebro-differential systems, implicit differential equations or singular systems.