21390624 has 108 divisors (see below), whose sum is σ = 64869714. Its totient is φ = 6684672.

The previous prime is 21390619. The next prime is 21390631. The reversal of 21390624 is 42609312.

21390624 is a `hidden beast` number, since 2 + 1 + 39 + 0 + 624 = 666.

It is a Cunningham number, because it is equal to 4625²-1.

It can be written as a sum of positive squares in 3 ways, for example, as 20250000 + 1140624 = 4500^2 + 1068^2 .

It is a hoax number, since the sum of its digits (27) coincides with the sum of the digits of its distinct prime factors.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 83104 + ... + 83360.

Almost surely, 2^21390624 is an apocalyptic number.

21390624 is a gapful number since it is divisible by the number (24) 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 21390624, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32434857).

21390624 is an abundant number, since it is smaller than the sum of its proper divisors (43479090).

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

21390624 is a wasteful number, since it uses less digits than its factorization.

21390624 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 307 (or 279 counting only the distinct ones).

The product of its (nonzero) digits is 2592, while the sum is 27.

The square root of 21390624 is about 4624.9998918919. Note that the first 3 decimals coincide. The cubic root of 21390624 is about 277.5925573889.

Adding to 21390624 its reverse (42609312), we get a palindrome (63999936).

The spelling of 21390624 in words is "twenty-one million, three hundred ninety thousand, six hundred twenty-four".