The entries of the Sinkhorn limit of an $m \times n$ matrix

We use Gr\"obner bases to compute the Sinkhorn limit of a positive $3 \times 3$ matrix $A$, showing that the entries are algebraic numbers with degree at most 6. The polynomial equation satisfied by each entry is large, but we show that it has a natural representation in terms of linear combinations of products of minors of $A$. We then use this representation to interpolate a polynomial equation satisfied by an entry of the Sinkhorn limit of a positive $4 \times 4$ matrix. Finally, we use the PSLQ algorithm and 1.5 years of CPU time to formulate a conjecture, up to certain signs that we have not been able to identify, for a polynomial equation satisfied by an entry of the Sinkhorn limit of a positive $m \times n$ matrix. In particular, we conjecture that the entries are algebraic numbers with degree at most $\binom{m + n - 2}{m - 1}$. This degree has a combinatorial interpretation as the number of minors of an $(m - 1) \times (n - 1)$ matrix, and the coefficients reflect new combinatorial structure on sets of minor specifications.
Comment: 25 pages; Mathematica package and documentation available as ancillary files