21917820 has 24 divisors (see below), whose sum is σ = 61370064. Its totient is φ = 5844736.

The previous prime is 21917813. The next prime is 21917843. The reversal of 21917820 is 2871912.

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

It is a congruent number.

It is an unprimeable number.

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 7 ways as a sum of consecutive naturals, for example, 182589 + ... + 182708.

It is an arithmetic number, because the mean of its divisors is an integer number (2557086).

Almost surely, 2^21917820 is an apocalyptic number.

21917820 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21917820 is an abundant number, since it is smaller than the sum of its proper divisors (39452244).

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

21917820 is a wasteful number, since it uses less digits than its factorization.

21917820 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 365309 (or 365307 counting only the distinct ones).

The product of its (nonzero) digits is 2016, while the sum is 30.

The square root of 21917820 is about 4681.6471460374. The cubic root of 21917820 is about 279.8546013811.

It can be divided in two parts, 2191 and 7820, that added together give a triangular number (10011 = T₁₄₁).

The spelling of 21917820 in words is "twenty-one million, nine hundred seventeen thousand, eight hundred twenty".