^Daniel Ratner; Manfred K. Warmuth. Finding a Shortest Solution for the N × N Extension of the 15-PUZZLE Is Intractable. National Conference on Artificial Intelligence. 1986.引文使用过时参数coauthors (帮助)
^Ratner, Daniel; Warmuth, Manfred. The (n2−1)-puzzle and related relocation problems. Journal of Symbolic Computation. 1990, 10 (2): 111–137. doi:10.1016/S0747-7171(08)80001-6.引文使用过时参数coauthors (帮助)