21155817 has 4 divisors (see below), whose sum is σ = 28207760. Its totient is φ = 14103876.

The previous prime is 21155789. The next prime is 21155839. The reversal of 21155817 is 71855112.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 21155817 - 2⁶ = 21155753 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21155887) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3525967 + ... + 3525972.

It is an arithmetic number, because the mean of its divisors is an integer number (7051940).

Almost surely, 2^21155817 is an apocalyptic number.

It is an amenable number.

21155817 is a deficient number, since it is larger than the sum of its proper divisors (7051943).

21155817 is an equidigital number, since it uses as much as digits as its factorization.

21155817 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7051942.

The product of its digits is 2800, while the sum is 30.

The square root of 21155817 is about 4599.5453036143. The cubic root of 21155817 is about 276.5730976628.

It can be divided in two parts, 211558 and 17, that added together give a triangular number (211575 = T₆₅₀).

The spelling of 21155817 in words is "twenty-one million, one hundred fifty-five thousand, eight hundred seventeen".