The only solutions are and (Ball and Coxeter 1987, Dickson 2005), as conjectured
by Lucas (1875), partially proved by Moret-Blanc (1876) and Lucas (1877), and proved
by Watson (1918). Watson's proof was almost elementary, disposing of most cases by
elementary means, but resorting to the use of elliptic functions for one pesky case.
Entirely elementary proofs have been given by Ma (1985) and Anglin (1990).
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