21015855 has 72 divisors (see below), whose sum is σ = 44068752. Its totient is φ = 9580032.

The previous prime is 21015851. The next prime is 21015857. The reversal of 21015855 is 55851012.

It is not a de Polignac number, because 21015855 - 2² = 21015851 is a prime.

It is a super-2 number, since 2×21015855² = 883332322762050, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21015851) by changing a digit.

It is a polite number, since it can be written in 71 ways as a sum of consecutive naturals, for example, 59359 + ... + 59711.

It is an arithmetic number, because the mean of its divisors is an integer number (612066).

Almost surely, 2^21015855 is an apocalyptic number.

21015855 is an abundant number, since it is smaller than the sum of its proper divisors (23052897).

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

21015855 is an equidigital number, since it uses as much as digits as its factorization.

21015855 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 387 (or 368 counting only the distinct ones).

The product of its (nonzero) digits is 2000, while the sum is 27.

The square root of 21015855 is about 4584.3052908811. The cubic root of 21015855 is about 275.9618330966.

Adding to 21015855 its reverse (55851012), we get a palindrome (76866867).

The spelling of 21015855 in words is "twenty-one million, fifteen thousand, eight hundred fifty-five".