81921 has 16 divisors (see below), whose sum is σ = 129024. Its totient is φ = 45264.

The previous prime is 81919. The next prime is 81929. The reversal of 81921 is 12918.

It is not a de Polignac number, because 81921 - 2¹ = 81919 is a prime.

It is a super-2 number, since 2×81921² = 13422100482, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (21).

It is not an unprimeable number, because it can be changed into a prime (81929) by changing a digit.

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 15 ways as a sum of consecutive naturals, for example, 946 + ... + 1028.

It is an arithmetic number, because the mean of its divisors is an integer number (8064).

It is a Proth number, since it is equal to 5 ⋅ 2¹⁴ + 1 and 5 < 2¹⁴.

2⁸¹⁹²¹ is an apocalyptic number.

It is an amenable number.

81921 is a deficient number, since it is larger than the sum of its proper divisors (47103).

81921 is a wasteful number, since it uses less digits than its factorization.

81921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 140.

The product of its digits is 144, while the sum is 21.

The square root of 81921 is about 286.2184480427. The cubic root of 81921 is about 43.4308585847.

The spelling of 81921 in words is "eighty-one thousand, nine hundred twenty-one".