Number 81 is not a prime number. It is a composite number.

A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. We do not consider 81 as a prime number, because it can be written as a product of two smaller natural numbers (check the factors of number 81 below).

Number of factors: 5.

List of factors/divisors: 1, 3, 9, 27, 81.

Parity: 81 is an odd number.

Perfect square: yes, because 81=9^{2} (a square number or perfect square is an integer that is the square of an integer).

Perfect number: no, because the sum of its proper divisors is 40 (perfect number is a positive integer that is equal to the sum of its proper divisors).

Number: Prime number: | 74 no | 75 no | 76 no | 77 no | 78 no | 79 yes | 80 no | 81 no | 82 no | 83 yes | 84 no | 85 no | 86 no | 87 no | 88 no |
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