21605723 has 4 divisors (see below), whose sum is σ = 21684000. Its totient is φ = 21527448.

The previous prime is 21605711. The next prime is 21605747. The reversal of 21605723 is 32750612.

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 21605723 - 2⁶ = 21605659 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21605692 and 21605701.

It is not an unprimeable number, because it can be changed into a prime (21605123) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 38723 + ... + 39276.

It is an arithmetic number, because the mean of its divisors is an integer number (5421000).

Almost surely, 2^21605723 is an apocalyptic number.

21605723 is a deficient number, since it is larger than the sum of its proper divisors (78277).

21605723 is an equidigital number, since it uses as much as digits as its factorization.

21605723 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 78276.

The product of its (nonzero) digits is 2520, while the sum is 26.

The square root of 21605723 is about 4648.1956714407. The cubic root of 21605723 is about 278.5199239679.

The spelling of 21605723 in words is "twenty-one million, six hundred five thousand, seven hundred twenty-three".