Sixty-one

Sixty-one (61, LXI) is the natural number following sixty and preceding sixty-two. It is the 18th prime number. The previous is fifty-nine, with which it comprises a twin prime. Sixty-one is the sum of two squares, 5² + 6².

Sixty-one is also:

• The atomic number of promethium, a lanthanide.
• The code for international direct dial phone calls to Australia.