21540665 has 4 divisors (see below), whose sum is σ = 25848804. Its totient is φ = 17232528.

The previous prime is 21540653. The next prime is 21540677. The reversal of 21540665 is 56604512.

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

It is an interprime number because it is at equal distance from previous prime (21540653) and next prime (21540677).

It can be written as a sum of positive squares in 2 ways, for example, as 4343056 + 17197609 = 2084^2 + 4147^2 .

It is a cyclic number.

It is not a de Polignac number, because 21540665 - 2¹⁴ = 21524281 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is an unprimeable number.

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, 2154062 + ... + 2154071.

It is an arithmetic number, because the mean of its divisors is an integer number (6462201).

Almost surely, 2^21540665 is an apocalyptic number.

It is an amenable number.

21540665 is a deficient number, since it is larger than the sum of its proper divisors (4308139).

21540665 is an equidigital number, since it uses as much as digits as its factorization.

21540665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4308138.

The product of its (nonzero) digits is 7200, while the sum is 29.

The square root of 21540665 is about 4641.1921959772. The cubic root of 21540665 is about 278.2400881272.

The spelling of 21540665 in words is "twenty-one million, five hundred forty thousand, six hundred sixty-five".