Pfaffian constraint

In dynamics, a Pfaffian constraint is a way to describe a dynamical system in the form:

${\displaystyle \sum _{s=1}^{n}A_{rs}du_{s}+A_{r}dt=0;\;r=1,\ldots ,L}$

where ${\displaystyle L}$ is the number of equations in a system of constraints, and ${\displaystyle A_{rs},A_{r}}$ are functions of ${\displaystyle t,u_{1},\dots ,u_{n}}$ only. In other words, it is a 1-form on ${\displaystyle \mathbb {R} ^{1+n}}$.