21991373 has 4 divisors (see below), whose sum is σ = 22001280. Its totient is φ = 21981468.

The previous prime is 21991349. The next prime is 21991397. The reversal of 21991373 is 37319912.

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, and also a brilliant number, because the two primes have the same length.

It is an interprime number because it is at equal distance from previous prime (21991349) and next prime (21991397).

It is a cyclic number.

It is not a de Polignac number, because 21991373 - 2⁸ = 21991117 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21991313) 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, 86 + ... + 6632.

It is an arithmetic number, because the mean of its divisors is an integer number (5500320).

Almost surely, 2^21991373 is an apocalyptic number.

It is an amenable number.

21991373 is a deficient number, since it is larger than the sum of its proper divisors (9907).

21991373 is an equidigital number, since it uses as much as digits as its factorization.

21991373 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9906.

The product of its digits is 10206, while the sum is 35.

The square root of 21991373 is about 4689.4960283596. The cubic root of 21991373 is about 280.1673022266.

The spelling of 21991373 in words is "twenty-one million, nine hundred ninety-one thousand, three hundred seventy-three".