21391820 has 12 divisors (see below), whose sum is σ = 44922864. Its totient is φ = 8556720.

The previous prime is 21391819. The next prime is 21391837. The reversal of 21391820 is 2819312.

It is a plaindrome in base 16.

It is a self number, because there is not a number n which added to its sum of digits gives 21391820.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 534776 + ... + 534815.

It is an arithmetic number, because the mean of its divisors is an integer number (3743572).

Almost surely, 2^21391820 is an apocalyptic number.

21391820 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21391820 is an abundant number, since it is smaller than the sum of its proper divisors (23531044).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21391820 is a wasteful number, since it uses less digits than its factorization.

21391820 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1069600 (or 1069598 counting only the distinct ones).

The product of its (nonzero) digits is 864, while the sum is 26.

The square root of 21391820 is about 4625.1291873849. The cubic root of 21391820 is about 277.5977309097.

The spelling of 21391820 in words is "twenty-one million, three hundred ninety-one thousand, eight hundred twenty".