21608731 has 4 divisors (see below), whose sum is σ = 21671352. Its totient is φ = 21546112.

The previous prime is 21608711. The next prime is 21608749. The reversal of 21608731 is 13780612.

21608731 = T₃₇₉₄ + T₃₇₉₅ + T₃₇₉₆.

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

It is a cyclic number.

It is not a de Polignac number, because 21608731 - 2⁷ = 21608603 is a prime.

It is a super-2 number, since 2×21608731² = 933874510860722, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21608731.

It is not an unprimeable number, because it can be changed into a prime (21608711) 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, 30790 + ... + 31483.

It is an arithmetic number, because the mean of its divisors is an integer number (5417838).

Almost surely, 2^21608731 is an apocalyptic number.

21608731 is the 3796-th centered triangular number.

21608731 is a deficient number, since it is larger than the sum of its proper divisors (62621).

21608731 is an equidigital number, since it uses as much as digits as its factorization.

21608731 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 62620.

The product of its (nonzero) digits is 2016, while the sum is 28.

The square root of 21608731 is about 4648.5192265925. The cubic root of 21608731 is about 278.5328487695.

The spelling of 21608731 in words is "twenty-one million, six hundred eight thousand, seven hundred thirty-one".