21457 has 4 divisors (see below), whose sum is σ = 22000. Its totient is φ = 20916.

The previous prime is 21433. The next prime is 21467. The reversal of 21457 is 75412.

21457 is nontrivially palindromic in base 12.

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 a de Polignac number, because none of the positive numbers 2^k-21457 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21407) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 207 + ... + 292.

It is an arithmetic number, because the mean of its divisors is an integer number (5500).

2²¹⁴⁵⁷ is an apocalyptic number.

It is an amenable number.

21457 is a deficient number, since it is larger than the sum of its proper divisors (543).

21457 is an equidigital number, since it uses as much as digits as its factorization.

21457 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 542.

The product of its digits is 280, while the sum is 19.

The square root of 21457 is about 146.4820808154. The cubic root of 21457 is about 27.7879388012.

Adding to 21457 its reverse (75412), we get a palindrome (96869).

The spelling of 21457 in words is "twenty-one thousand, four hundred fifty-seven".