• 81 can be written using four 4's:

81 has 5 divisors (see below), whose sum is σ = 121. Its totient is φ = 54.

The previous prime is 79. The next prime is 83. The reversal of 81 is 18.

81 = T_{8} + T_{9}.

The square root of 81 is 9.

It is a perfect power (a square, a biquadrate), and thus also a powerful number.

81 is nontrivially palindromic in base 8.

81 is an esthetic number in base 8 and base 15, because in such bases its adjacent digits differ by 1.

It is an interprime number because it is at equal distance from previous prime (79) and next prime (83).

It is an ABA number since it can be written as A⋅B^{A}, here for A=3, B=3.

It is not a de Polignac number, because 81 - 2^{1} = 79 is a prime.

It is a super-2 number, since 2×81^{2} = 13122, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (9).

It is a tribonacci number.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

81 is an undulating number in base 8.

It is a Curzon number.

It is a plaindrome in base 7, base 12, base 14 and base 15.

It is a nialpdrome in base 3, base 5, base 9, base 10, base 11, base 13 and base 16.

It is a panconsummate number.

It is a pernicious number, because its binary representation contains a prime number (3) of ones.

It is a polite number, since it can be written in 4 ways as a sum of consecutive naturals, for example, 26 + 27 + 28.

It is a Proth number, since it is equal to 5 ⋅ 2^{4} + 1 and 5 < 2^{4}.

81 is the 9-th square number and also the 6-th heptagonal number.

81 is the 5-th centered octagonal number.

It is an amenable number.

81 is a deficient number, since it is larger than the sum of its proper divisors (40).

81 is an equidigital number, since it uses as much as digits as its factorization.

81 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 12 (or 3 counting only the distinct ones).

The product of its digits is 8, while the sum is 9.

The cubic root of 81 is about 4.3267487109.

Adding to 81 its reverse (18), we get a palindrome (99).

The spelling of 81 in words is "eighty-one", and thus it is an aban number and an uban number.

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