81 (number)

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81 (eighty-one) is the natural number following 80 and preceding 82.


Cardinal 81
eighty-one
Ordinal 81st
eighty-first
Factorization 34
Divisors 1, 3, 9, 27, 81
Roman numeral LXXXI
Binary 01010001
Hexadecimal 51
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[edit] In mathematics

Eighty-one is the square of 9 and the fourth power of 3. Like all powers of three, 81 is a perfect totient number It is a heptagonal number and a centered octagonal number. It is also a tribonacci number, an open meandric number. 81 is the ninth member of the Mian-Chowla sequence.

In base 10, it is a Harshad number, and one of three numbers (the other two are 1458 and 1729) which, when its digits are added together, produces a sum which, when multiplied by its reversed self, yields the original number:

1 + 8 = 9
9 · 9 = 81

(although this case is somewhat degenerate, as the sum has only a single digit).

[edit] In astronomy

[edit] In other fields

Eighty-one is also:

[edit] In Culture

The Arabic numerals for 81 are visible in the left palm of the human hand.