Let $r \leqslant n$ be nonnegative integers, and let $N = \binom{ n}{ r} - 1$. For a matroid $M$ of rank $r$ on the finite set $E = [n]$ and a partial field $k$ in the sense of Semple--Whittle, it is known that the following are equivalent: (a) $M$ is representable over $k$; (b) there is a point $p = (p_J) \in { \bf P}^N(k)$ with support $M$ (meaning that $\text{ Supp}(p) := \{ J \in \binom{ E}{ r} \; \vert \; p_J \ne 0\}$ of $p$ is the set of bases of $M$) satisfying the Grassmann-Pl\"ucker equations; and (c) there is a point $p = (p_J) \in { \bf P}^N(k)$ with support $M$ satisfying just the 3-term Grassmann-Pl\"ucker equations. Moreover, by a theorem of P. Nelson, almost all matroids (meaning asymptotically 100%) are not representable over any partial field. We prove analogues of these facts for Lagrangian orthogonal matroids in the sense of Gelfand-Serganova, which are equivalent to even Delta-matroids in the sense of Bouchet.
Categories: math.CO