Counting Near-Spanning Matchings in Latin Squares and Steiner Triple Systems

Abstract: Montgomery recently proved that for sufficiently large $n$, every Latin square of order $n$ has a partial transversal with $n-1$ cells, and every Steiner triple system of order $n$ has a matching with $\lfloor n/3\rfloor-1$ edges, thus confirming the Ryser--Brualdi--Stein conjecture for even $n$ and the conjecture of Brouwer. We prove sharp enumerative refinements of these results: there is an absolute constant $c&gt;0$ such that, for sufficiently large $n$, 1) every Latin square of order $n$ has $$\left((1\pm n^{{-c})\frac{n}{\mathrm} {e}<sup>2}\right)<sup>n$$ partial transversals with $n-1$ cells; 2) every Steiner triple system of order $n$ has $$\left((1\pm n^{{-c})\frac{n}{2\mathrm} {e}^{2}\right)^{\lfloor} n/3\rfloor}$$ matchings with $\lfloor n/3\rfloor-1$ edges. The first estimate confirms predictions of Montgomery and Kelly.