Forty-one (41, LXI) is the natural number following forty and preceding forty-two.

Forty-one is the 13th smallest prime number. The next is forty-three, with which it comprises a twin prime. Forty-one is also a Sophie Germain prime.

The number figures in the polynomial f(n) = n² - n + 41, which yields primes for 0 < n < 41. Forty-one in the sum of two squares, 4² + 5², as well as the sum of the first six prime numbers, and also the sum of three primes (11 + 13 + 17).

Forty-one is also:

• The atomic number of niobium.
• In A Thousand and One Nights, the number of thieves accompanying Ali Baba.
• A number 'encoded' in a lot of the music of Johann Sebastian Bach. Bach may have considered this number a sort of signature, since given A = 1, B = 2, C = 3, etc., then B + A + C + H = 14 and J + S + B + A + C + H = 41.
• The code for direct dial international phone calls to Switzerland.