21165805 has 4 divisors (see below), whose sum is σ = 25398972. Its totient is φ = 16932640.

The previous prime is 21165787. The next prime is 21165811. The reversal of 21165805 is 50856112.

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

It can be written as a sum of positive squares in 2 ways, for example, as 14845609 + 6320196 = 3853^2 + 2514^2 .

It is not a de Polignac number, because 21165805 - 2¹⁷ = 21034733 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, 2116576 + ... + 2116585.

It is an arithmetic number, because the mean of its divisors is an integer number (6349743).

Almost surely, 2^21165805 is an apocalyptic number.

It is an amenable number.

21165805 is a deficient number, since it is larger than the sum of its proper divisors (4233167).

21165805 is an equidigital number, since it uses as much as digits as its factorization.

21165805 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4233166.

The product of its (nonzero) digits is 2400, while the sum is 28.

The square root of 21165805 is about 4600.6309349914. The cubic root of 21165805 is about 276.6166156778.

It can be divided in two parts, 2116 and 5805, that added together give a square (7921 = 89²).

The spelling of 21165805 in words is "twenty-one million, one hundred sixty-five thousand, eight hundred five".