Seventyone

Seventy-one is the positive integer that follows seventy and precedes seventy-two. In words it is written as seventy-one; in hyphenated form when used as an adjective, it is seventy-one. The name is formed from the word seventy, itself derived from seven and the Old English suffix meaning tens.

In mathematics, seventy-one is a prime number, meaning it has no positive divisors other than 1 and itself. It is the 20th prime. It is odd and greater than two. In different bases, 71 is written as 1000111 in binary and as 47 in hexadecimal. Because 71 is congruent to 3 modulo 4, it remains a Gaussian prime; it cannot be expressed as a sum of two squares.

In science and numerals, 71 is the atomic number of lutetium (Lu), the heaviest naturally occurring lanthanide. The element’s name derives from Lutetia, the Latin name for Paris. The Roman numeral for 71 is LXXI. Messier 71 is a globular cluster in the constellation Sagitta, listed in the Messier catalog.

In culture, 71 is the title of a 2014 British thriller film directed by Yann Demange, set during the Troubles in Northern Ireland. The number also appears in various contexts, such as anniversaries, sports jerseys, and geographic designations.