64 (number) 64 (number) 64 (sixty-four) is the natural number following 63 and preceding 65. Sixty-four is the square of 8, the cube of 4, and the sixth power of 2. It is the smallest number with exactly seven divisors. It is the lowest positive power of two that is adjacent to neither a Mersenne prime nor a Fermat prime. 64 is the sum of Euler's totient function for the first fourteen integers. It is also a dodecagonal number and a centered triangular number. 64 is also the first whole number that is both a perfect square and a perfect cube. Since
Integer triangle double triangular number ( + 1) and for "c" odd it is the square . It also means that the number of integer triangles with greatest side "c" exceeds the number of integer triangles with greatest side "c"−2 by "c". The sequence of the number of non-congruent integer triangles with largest side "c", starting at "c" = 1, is: The number of integer triangles (up to congruence) with given largest side "c" and integer triple ("a", "b", "c") that lie on or within a semicircle of diameter "c" is the number of integer triples such that "a" + "b" >