21891309 has 16 divisors (see below), whose sum is σ = 32704320. Its totient is φ = 12908160.

The previous prime is 21891293. The next prime is 21891319. The reversal of 21891309 is 90319812.

It is not a de Polignac number, because 21891309 - 2⁴ = 21891293 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (33).

It is a self number, because there is not a number n which added to its sum of digits gives 21891309.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21891319) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 7744 + ... + 10185.

It is an arithmetic number, because the mean of its divisors is an integer number (2044020).

Almost surely, 2^21891309 is an apocalyptic number.

It is an amenable number.

21891309 is a deficient number, since it is larger than the sum of its proper divisors (10813011).

21891309 is a wasteful number, since it uses less digits than its factorization.

21891309 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 17980.

The product of its (nonzero) digits is 3888, while the sum is 33.

The square root of 21891309 is about 4678.8149140568. The cubic root of 21891309 is about 279.7417218659.

The spelling of 21891309 in words is "twenty-one million, eight hundred ninety-one thousand, three hundred nine".