21523709 has 4 divisors (see below), whose sum is σ = 21574080. Its totient is φ = 21473340.

The previous prime is 21523681. The next prime is 21523727. The reversal of 21523709 is 90732512.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21523709 - 2²⁴ = 4746493 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21523739) 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, 24539 + ... + 25400.

It is an arithmetic number, because the mean of its divisors is an integer number (5393520).

Almost surely, 2^21523709 is an apocalyptic number.

It is an amenable number.

21523709 is a deficient number, since it is larger than the sum of its proper divisors (50371).

21523709 is an equidigital number, since it uses as much as digits as its factorization.

21523709 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 50370.

The product of its (nonzero) digits is 3780, while the sum is 29.

The square root of 21523709 is about 4639.3651505351. The cubic root of 21523709 is about 278.1670622577.

The spelling of 21523709 in words is "twenty-one million, five hundred twenty-three thousand, seven hundred nine".