81 (number)
81 (eighty-one) is the natural number following 80 and preceding 82.
| ||||
|---|---|---|---|---|
| Cardinal | eighty-first | |||
| Ordinal | st | |||
| Numeral system | 81 | |||
| Factorization | ||||
| Divisors | 1, 3, 9, 27, 81 | |||
| Greek numeral | ΠΑ´ | |||
| Roman numeral | LXXXI | |||
| Binary | 10100012 | |||
| Ternary | 100003 | |||
| Senary | 2136 | |||
| Octal | 1218 | |||
| Duodecimal | 6912 | |||
| Hexadecimal | 5116 | |||
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, and 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 non-trivial 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:
- 8 + 1 = 9
- 9 X 9 = 81
(although this case is somewhat degenerate, as the sum has only a single digit).
The inverse of 81 is 0.012345679 recurring, tantalisingly close to being a repeating set of all the digits 0-9 successively. The full sequence is reserved for the significantly more cumbersome fraction 13,717,421/1,111,111,111.
In other fields
Eighty-one is also:
- The atomic number of thallium
- The code for international direct dial phone calls to Japan