21666665 has 8 divisors (see below), whose sum is σ = 26490888. Its totient is φ = 17006080.

The previous prime is 21666649. The next prime is 21666677. The reversal of 21666665 is 56666612.

It can be written as a sum of positive squares in 4 ways, for example, as 3279721 + 18386944 = 1811^2 + 4288^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21666665 - 2⁴ = 21666649 is a prime.

It is a super-4 number, since 4×21666665⁴ (a number of 30 digits) contains 4444 as substring.

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 7 ways as a sum of consecutive naturals, for example, 40616 + ... + 41145.

It is an arithmetic number, because the mean of its divisors is an integer number (3311361).

Almost surely, 2^21666665 is an apocalyptic number.

It is an amenable number.

21666665 is a deficient number, since it is larger than the sum of its proper divisors (4824223).

21666665 is an equidigital number, since it uses as much as digits as its factorization.

21666665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 81819.

The product of its digits is 77760, while the sum is 38.

The square root of 21666665 is about 4654.7465022276. The cubic root of 21666665 is about 278.7815464307.

The spelling of 21666665 in words is "twenty-one million, six hundred sixty-six thousand, six hundred sixty-five".