21509855 has 4 divisors (see below), whose sum is σ = 25811832. Its totient is φ = 17207880.

The previous prime is 21509849. The next prime is 21509867. The reversal of 21509855 is 55890512.

It is a semiprime because it is the product of two primes.

It is not a de Polignac number, because 21509855 - 2⁸ = 21509599 is a prime.

It is a Duffinian number.

It is a congruent number.

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, 2150981 + ... + 2150990.

It is an arithmetic number, because the mean of its divisors is an integer number (6452958).

Almost surely, 2^21509855 is an apocalyptic number.

21509855 is a deficient number, since it is larger than the sum of its proper divisors (4301977).

21509855 is an equidigital number, since it uses as much as digits as its factorization.

21509855 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4301976.

The product of its (nonzero) digits is 18000, while the sum is 35.

The square root of 21509855 is about 4637.8718179786. The cubic root of 21509855 is about 278.1073675646.

The spelling of 21509855 in words is "twenty-one million, five hundred nine thousand, eight hundred fifty-five".