Sixty-one

61 (sixty-one) is the natural number following 60 and preceding 62.

List of numbers — Integers << 60 61 62 63 64 65 66 67 68 69 >>
Cardinal	sixty-one
Ordinal	61st (sixty-first_
Factorization	prime
Roman numeral	LXI
Binary	111101
Hexadecimal	3D

Mathematics

It is the 18th prime number. The previous is 59, with which it comprises a twin prime. Sixty-one is a cuban prime of the form <math>p = (x^3 - y^3) / (x - y), x = y + 1<math>. Sixty-one is the sum of two squares, 5² + 6², and it is also a centered square number and a centered hexagonal number.

It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its base 10 digits: 6, 1, 7, 8, 15, 23, 38, 61...

Other fields

Sixty-one is:

• The atomic number of promethium, a lanthanide.
• The number of the French department Orne.
• The code for international direct dial phone calls to Australia.
• 61*, a 2001 baseball movie directed by Billy Crystal.
• The year AD 61, 61 BC, or 1961.