21808521 has 24 divisors (see below), whose sum is σ = 37337664. Its totient is φ = 12461040.

The previous prime is 21808519. The next prime is 21808529. The reversal of 21808521 is 12580812.

It is not a de Polignac number, because 21808521 - 2¹ = 21808519 is a prime.

It is a super-2 number, since 2×21808521² = 951223176414882, which contains 22 as substring.

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

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21808529) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 4710 + ... + 8111.

It is an arithmetic number, because the mean of its divisors is an integer number (1555736).

Almost surely, 2^21808521 is an apocalyptic number.

21808521 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21808521 is a deficient number, since it is larger than the sum of its proper divisors (15529143).

21808521 is an equidigital number, since it uses as much as digits as its factorization.

21808521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 12843 (or 12831 counting only the distinct ones).

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

The square root of 21808521 is about 4669.9594216652. The cubic root of 21808521 is about 279.3886363327.

It can be divided in two parts, 2180 and 8521, that added together give a palindrome (10701).

The spelling of 21808521 in words is "twenty-one million, eight hundred eight thousand, five hundred twenty-one".