Eighty-one

Eighty-one (81, LXXXI) is the natural number following eighty and preceeding eighty-two.

• Hexadecimal: 51
• Binary: 01010001
• Factors: 1, 3, 9, 27, 81
• Factorization:
• Ordinal number: eighty-first

81 is also:

• the atomic number of thallium (Tl)
• the title of a short film by Stephen Burke: 81
• the model number of: 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 year AD 81 or AD 1981.