21897 has 8 divisors (see below), whose sum is σ = 32480. Its totient is φ = 14580.

The previous prime is 21893. The next prime is 21911. The reversal of 21897 is 79812.

It is not a de Polignac number, because 21897 - 2² = 21893 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 811 = 21897 / (2 + 1 + 8 + 9 + 7).

It is a Duffinian number.

21897 is a lucky number.

It is a plaindrome in base 16.

It is a self number, because there is not a number n which added to its sum of digits gives 21897.

It is not an unprimeable number, because it can be changed into a prime (21893) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 379 + ... + 432.

It is an arithmetic number, because the mean of its divisors is an integer number (4060).

2²¹⁸⁹⁷ is an apocalyptic number.

21897 is a gapful number since it is divisible by the number (27) formed by its first and last digit.

It is an amenable number.

21897 is a deficient number, since it is larger than the sum of its proper divisors (10583).

21897 is an equidigital number, since it uses as much as digits as its factorization.

21897 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 820 (or 814 counting only the distinct ones).

The product of its digits is 1008, while the sum is 27.

The square root of 21897 is about 147.9763494617. The cubic root of 21897 is about 27.9765960895.

The spelling of 21897 in words is "twenty-one thousand, eight hundred ninety-seven".