21081864 has 16 divisors (see below), whose sum is σ = 52704720. Its totient is φ = 7027280.

The previous prime is 21081859. The next prime is 21081877. The reversal of 21081864 is 46818012.

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, 439182 + ... + 439229.

It is an arithmetic number, because the mean of its divisors is an integer number (3294045).

Almost surely, 2^21081864 is an apocalyptic number.

21081864 is a gapful number since it is divisible by the number (24) formed by its first and last digit.

It is an amenable number.

21081864 is an abundant number, since it is smaller than the sum of its proper divisors (31622856).

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

21081864 is a wasteful number, since it uses less digits than its factorization.

21081864 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 878420 (or 878416 counting only the distinct ones).

The product of its (nonzero) digits is 3072, while the sum is 30.

The square root of 21081864 is about 4591.4991016007. The cubic root of 21081864 is about 276.2504552861.

Adding to 21081864 its reverse (46818012), we get a palindrome (67899876).

It can be divided in two parts, 21081 and 864, that added together give a triangular number (21945 = T₂₀₉).

The spelling of 21081864 in words is "twenty-one million, eighty-one thousand, eight hundred sixty-four".