21191820 has 96 divisors (see below), whose sum is σ = 64774080. Its totient is φ = 5145600.

The previous prime is 21191813. The next prime is 21191831. The reversal of 21191820 is 2819112.

It is an unprimeable number.

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 31 ways as a sum of consecutive naturals, for example, 78646 + ... + 78914.

It is an arithmetic number, because the mean of its divisors is an integer number (674730).

Almost surely, 2^21191820 is an apocalyptic number.

21191820 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21191820, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32387040).

21191820 is an abundant number, since it is smaller than the sum of its proper divisors (43582260).

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

21191820 is a wasteful number, since it uses less digits than its factorization.

21191820 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 395 (or 393 counting only the distinct ones).

The product of its (nonzero) digits is 288, while the sum is 24.

The square root of 21191820 is about 4603.4573963490. The cubic root of 21191820 is about 276.7298995805.

The spelling of 21191820 in words is "twenty-one million, one hundred ninety-one thousand, eight hundred twenty".