21805257 has 12 divisors (see below), whose sum is σ = 29773104. Its totient is φ = 14195160.

The previous prime is 21805229. The next prime is 21805283. The reversal of 21805257 is 75250812.

It is not a de Polignac number, because 21805257 - 2⁸ = 21805001 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21805297) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 3582 + ... + 7512.

It is an arithmetic number, because the mean of its divisors is an integer number (2481092).

Almost surely, 2^21805257 is an apocalyptic number.

It is an amenable number.

21805257 is a deficient number, since it is larger than the sum of its proper divisors (7967847).

21805257 is an equidigital number, since it uses as much as digits as its factorization.

21805257 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4020 (or 3977 counting only the distinct ones).

The product of its (nonzero) digits is 5600, while the sum is 30.

The square root of 21805257 is about 4669.6099408837. The cubic root of 21805257 is about 279.3746972856.

The spelling of 21805257 in words is "twenty-one million, eight hundred five thousand, two hundred fifty-seven".