21921657 has 4 divisors (see below), whose sum is σ = 29228880. Its totient is φ = 14614436.

The previous prime is 21921637. The next prime is 21921659. The reversal of 21921657 is 75612912.

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 21921657 - 2²⁴ = 5144441 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21921659) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3653607 + ... + 3653612.

It is an arithmetic number, because the mean of its divisors is an integer number (7307220).

Almost surely, 2^21921657 is an apocalyptic number.

It is an amenable number.

21921657 is a deficient number, since it is larger than the sum of its proper divisors (7307223).

21921657 is an equidigital number, since it uses as much as digits as its factorization.

21921657 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7307222.

The product of its digits is 7560, while the sum is 33.

The square root of 21921657 is about 4682.0569197736. The cubic root of 21921657 is about 279.8709311598.

It can be divided in two parts, 21921 and 657, that added together give a triangular number (22578 = T₂₁₂).

The spelling of 21921657 in words is "twenty-one million, nine hundred twenty-one thousand, six hundred fifty-seven".