81 (number)

81 is the natural number following 80 and preceding 82.

List of numbers — Integers << 80 81 82 83 84 85 86 87 88 89 >>
Cardinal	eighty-one
Ordinal	81st (eighty-first)
Factorization	<math>3^4<math>
Divisors	3, 9, 27
Roman numeral	LXXXI
Binary	01010001
Hexadecimal	51

Eighty-one is a square and a heptagonal number. It is also a tribonacci number, an open meandric number and a Harshad number.

Eighty-one is also:

• the atomic number of thallium (Tl)
• the title of a short film by Stephen Burke: Unterseeboot 81, Sinclair ZX81
• the Messier number of the M81 Group of galaxies
• the number of the French department Tarn
• the designation of Interstate 81, a freeway that runs from Tennessee to New York.
• the code for international direct dial phone calls to Japan
• the number of times a popular rhyme (erroneously) claims Lizzie Borden struck her parents with an axe
• The year AD 81, 81 BC, or 1981.

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