A problem that recently arose on
sci.math was the (well-known) problem
of finding when triangular numbers are square numbers; the first few quickly lead to
, as can be determined with a few minutes
computation. This is sequence M4536 in the book,
and references are provided to Dickson's History, to Beiler and other recreational mathematics books.
In some sense this is what the book is for: to give people an index into
what is `well-known' and perhaps to avoid ingenious but ultimately