21511855 has 4 divisors (see below), whose sum is σ = 25814232. Its totient is φ = 17209480.

The previous prime is 21511849. The next prime is 21511871. The reversal of 21511855 is 55811512.

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

It is not a de Polignac number, because 21511855 - 2²³ = 13123247 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, 2151181 + ... + 2151190.

It is an arithmetic number, because the mean of its divisors is an integer number (6453558).

Almost surely, 2^21511855 is an apocalyptic number.

21511855 is a deficient number, since it is larger than the sum of its proper divisors (4302377).

21511855 is an equidigital number, since it uses as much as digits as its factorization.

21511855 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4302376.

The product of its digits is 2000, while the sum is 28.

The square root of 21511855 is about 4638.0874291026. The cubic root of 21511855 is about 278.1159868308.

It can be divided in two parts, 21511 and 855, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21511855 in words is "twenty-one million, five hundred eleven thousand, eight hundred fifty-five".