Pfaffian Form

A 1-form

$$\omega=\sum_{i=1}^n a_i(x)\,dx_i$$

such that

$$\omega=0.$$

References

Knuth, D. E. ``Overlapping Pfaffians.'' Electronic J. Combinatorics 3, No. 2, R5, 1-13, 1996. http://www.combinatorics.org/Volume_3/volume3_2.html#R5.