21357991 has 8 divisors (see below), whose sum is σ = 22297184. Its totient is φ = 20437920.

The previous prime is 21357983. The next prime is 21357997. The reversal of 21357991 is 19975312.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21357991 - 2³ = 21357983 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (37).

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 21357991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21357997) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2475 + ... + 6988.

It is an arithmetic number, because the mean of its divisors is an integer number (2787148).

Almost surely, 2^21357991 is an apocalyptic number.

21357991 is a deficient number, since it is larger than the sum of its proper divisors (939193).

21357991 is an equidigital number, since it uses as much as digits as its factorization.

21357991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9561.

The product of its digits is 17010, while the sum is 37.

The square root of 21357991 is about 4621.4706533743. The cubic root of 21357991 is about 277.4513227932.

The spelling of 21357991 in words is "twenty-one million, three hundred fifty-seven thousand, nine hundred ninety-one".