The knight's tour problem
The knight's tour problem is this: can a knight visit all the squares of the board exactly once and return to its starting position? In this paper we use the word tour in the sense of a closed or re-entrant tour. Some authors use the word tour in the more general sense of an open tour-one not requiring the knight to return to its initial position, and some authors refer to closed tours as circuits. As documented by Murray [52] (see also [53] and [68]), the knight's tour problem dates back over a thousand years to Indian chess and has numerous appearances throughout the history of the game of chess (but not back as far as 200 BCE, as some have claimed [73, 74]). The prob lem was investigated by mathematicians such as Euler [2] and Vandermonde [71]; in modern terminology, the tour is an example of a Hamiltonian circuit [80], [5, Chap ter 11]. There is a vast literature on the problem. As Kraitchik remarked [42] (in a paper first published in 1941), "Many generalizations of the knight's problem have been proposed. Many alterations of the size and shape of the board have already been considered."