The $n$-queens completion problem
This addresses a classic combinatorial puzzle with implications for extremal graph theory and constraint satisfaction, but it is incremental in extending known bounds.
The paper tackles the n-queens completion problem by determining the minimum size of a partial configuration that can always be completed, showing that any placement of at most n/60 mutually non-attacking queens can be completed, while some configurations of roughly n/4 queens cannot.
An $n$-queens configuration is a placement of $n$ mutually non-attacking queens on an $n\times n$ chessboard. The $n$-queens completion problem, introduced by Nauck in 1850, is to decide whether a given partial configuration can be completed to an $n$-queens configuration. In this paper, we study an extremal aspect of this question, namely: how small must a partial configuration be so that a completion is always possible? We show that any placement of at most $n/60$ mutually non-attacking queens can be completed. We also provide partial configurations of roughly $n/4$ queens that cannot be completed, and formulate a number of interesting problems. Our proofs connect the queens problem to rainbow matchings in bipartite graphs and use probabilistic arguments together with linear programming duality.