21808251 has 16 divisors (see below), whose sum is σ = 32389920. Its totient is φ = 14502240.

The previous prime is 21808247. The next prime is 21808271. The reversal of 21808251 is 15280812.

It is not a de Polignac number, because 21808251 - 2² = 21808247 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is not an unprimeable number, because it can be changed into a prime (21808231) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 13861 + ... + 15353.

It is an arithmetic number, because the mean of its divisors is an integer number (2024370).

Almost surely, 2^21808251 is an apocalyptic number.

21808251 is a deficient number, since it is larger than the sum of its proper divisors (10581669).

21808251 is a wasteful number, since it uses less digits than its factorization.

21808251 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2043 (or 2037 counting only the distinct ones).

The product of its (nonzero) digits is 1280, while the sum is 27.

The square root of 21808251 is about 4669.9305134017. The cubic root of 21808251 is about 279.3874833393.

The spelling of 21808251 in words is "twenty-one million, eight hundred eight thousand, two hundred fifty-one".