21057 has 4 divisors (see below), whose sum is σ = 28080. Its totient is φ = 14036.

The previous prime is 21031. The next prime is 21059. The reversal of 21057 is 75012.

It is a happy number.

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 a cyclic number.

It is not a de Polignac number, because 21057 - 2⁷ = 20929 is a prime.

It is a D-number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21057.

It is not an unprimeable number, because it can be changed into a prime (21059) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3507 + ... + 3512.

It is an arithmetic number, because the mean of its divisors is an integer number (7020).

2²¹⁰⁵⁷ is an apocalyptic number.

It is an amenable number.

21057 is a deficient number, since it is larger than the sum of its proper divisors (7023).

21057 is an equidigital number, since it uses as much as digits as its factorization.

21057 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7022.

The product of its (nonzero) digits is 70, while the sum is 15.

The square root of 21057 is about 145.1103028734. The cubic root of 21057 is about 27.6141809083.

Adding to 21057 its reverse (75012), we get a palindrome (96069).

It can be divided in two parts, 2105 and 7, that added together give a palindrome (2112).

The spelling of 21057 in words is "twenty-one thousand, fifty-seven".