21155555 has 4 divisors (see below), whose sum is σ = 25386672. Its totient is φ = 16924440.

The previous prime is 21155551. The next prime is 21155591. The reversal of 21155555 is 55555112.

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

It is not a de Polignac number, because 21155555 - 2² = 21155551 is a prime.

It is a Duffinian number.

21155555 is a modest number, since divided by 5555 gives 2115 as remainder.

It is not an unprimeable number, because it can be changed into a prime (21155551) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2115551 + ... + 2115560.

It is an arithmetic number, because the mean of its divisors is an integer number (6346668).

Almost surely, 2^21155555 is an apocalyptic number.

21155555 is a deficient number, since it is larger than the sum of its proper divisors (4231117).

21155555 is an equidigital number, since it uses as much as digits as its factorization.

21155555 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4231116.

The product of its digits is 6250, while the sum is 29.

The square root of 21155555 is about 4599.5168224499. The cubic root of 21155555 is about 276.5719559366.

The spelling of 21155555 in words is "twenty-one million, one hundred fifty-five thousand, five hundred fifty-five".