Balanced player arrangement in rotating court games

Abstract

Consider a game involving a team with $n$ players, $k$ of which wear shirts marked with a letter $A$, while the others with a letter $B$, and such that only $s$ people play, while the remaining $n-s$ wait outside the court. At certain times the players rotate, and one player enters the court while another one leaves, each player keeping the same neighbors at all times. The house rules want at least $t$ players wearing a shirt with the letter $A$ in the court at each rotation. We show that this can be achieved if and only if $nt\leq ks$. We use classical work on balanced words dating back to Christoffel and Smith.