21721391 has 4 divisors (see below), whose sum is σ = 21877800. Its totient is φ = 21564984.

The previous prime is 21721387. The next prime is 21721397. The reversal of 21721391 is 19312712.

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 21721391 - 2² = 21721387 is a prime.

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 21721391.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21721397) 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, 77996 + ... + 78273.

It is an arithmetic number, because the mean of its divisors is an integer number (5469450).

Almost surely, 2^21721391 is an apocalyptic number.

21721391 is a deficient number, since it is larger than the sum of its proper divisors (156409).

21721391 is a wasteful number, since it uses less digits than its factorization.

21721391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 156408.

The product of its digits is 756, while the sum is 26.

The square root of 21721391 is about 4660.6213105122. The cubic root of 21721391 is about 279.0160660147.

The spelling of 21721391 in words is "twenty-one million, seven hundred twenty-one thousand, three hundred ninety-one".