21537991 has 4 divisors (see below), whose sum is σ = 21561232. Its totient is φ = 21514752.

The previous prime is 21537979. The next prime is 21538003. The reversal of 21537991 is 19973512.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21537979) and next prime (21538003).

It is a cyclic number.

It is not a de Polignac number, because 21537991 - 2⁷ = 21537863 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 21537991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21537961) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 10170 + ... + 12103.

It is an arithmetic number, because the mean of its divisors is an integer number (5390308).

Almost surely, 2^21537991 is an apocalyptic number.

21537991 is a deficient number, since it is larger than the sum of its proper divisors (23241).

21537991 is an equidigital number, since it uses as much as digits as its factorization.

21537991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 23240.

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

The square root of 21537991 is about 4640.9041145018. The cubic root of 21537991 is about 278.2285743263.

The spelling of 21537991 in words is "twenty-one million, five hundred thirty-seven thousand, nine hundred ninety-one".