21657 has 4 divisors (see below), whose sum is σ = 28880. Its totient is φ = 14436.

The previous prime is 21649. The next prime is 21661. The reversal of 21657 is 75612.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 21657 - 2³ = 21649 is a prime.

It is a D-number.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21617) 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 3 ways as a sum of consecutive naturals, for example, 3607 + ... + 3612.

It is an arithmetic number, because the mean of its divisors is an integer number (7220).

2²¹⁶⁵⁷ is an apocalyptic number.

It is an amenable number.

21657 is a deficient number, since it is larger than the sum of its proper divisors (7223).

21657 is an equidigital number, since it uses as much as digits as its factorization.

21657 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7222.

The product of its digits is 420, while the sum is 21.

The square root of 21657 is about 147.1631747415. The cubic root of 21657 is about 27.8740087591.

The spelling of 21657 in words is "twenty-one thousand, six hundred fifty-seven".