21991800 has 48 divisors (see below), whose sum is σ = 68176440. Its totient is φ = 5864320.

The previous prime is 21991799. The next prime is 21991807. The reversal of 21991800 is 819912.

It is a Harshad number since it is a multiple of its sum of digits (30).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21991807) 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 11 ways as a sum of consecutive naturals, for example, 17727 + ... + 18926.

Almost surely, 2^21991800 is an apocalyptic number.

21991800 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21991800 is an abundant number, since it is smaller than the sum of its proper divisors (46184640).

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

21991800 is a wasteful number, since it uses less digits than its factorization.

21991800 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 36672 (or 36663 counting only the distinct ones).

The product of its (nonzero) digits is 1296, while the sum is 30.

The square root of 21991800 is about 4689.5415554188. The cubic root of 21991800 is about 280.1691155234.

It can be divided in two parts, 21991 and 800, that added together give a triangular number (22791 = T₂₁₃).

The spelling of 21991800 in words is "twenty-one million, nine hundred ninety-one thousand, eight hundred".