21957 has 8 divisors (see below), whose sum is σ = 31584. Its totient is φ = 13488.

The previous prime is 21943. The next prime is 21961. The reversal of 21957 is 75912.

21957 is nontrivially palindromic in base 11.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21957 - 2⁶ = 21893 is a prime.

21957 is a lucky number.

It is a plaindrome in base 15.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21957.

It is not an unprimeable number, because it can be changed into a prime (21937) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 243 + ... + 320.

It is an arithmetic number, because the mean of its divisors is an integer number (3948).

2²¹⁹⁵⁷ is an apocalyptic number.

It is an amenable number.

21957 is a deficient number, since it is larger than the sum of its proper divisors (9627).

21957 is a wasteful number, since it uses less digits than its factorization.

21957 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 579.

The product of its digits is 630, while the sum is 24.

The square root of 21957 is about 148.1789458729. The cubic root of 21957 is about 28.0021256890.

It can be divided in two parts, 219 and 57, that added together give a triangular number (276 = T₂₃).

The spelling of 21957 in words is "twenty-one thousand, nine hundred fifty-seven".