21745891 has 4 divisors (see below), whose sum is σ = 21756672. Its totient is φ = 21735112.

The previous prime is 21745799. The next prime is 21745901. The reversal of 21745891 is 19854712.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21745891 - 2⁹ = 21745379 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 21745891.

It is not an unprimeable number, because it can be changed into a prime (21745991) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1360 + ... + 6733.

It is an arithmetic number, because the mean of its divisors is an integer number (5439168).

Almost surely, 2^21745891 is an apocalyptic number.

21745891 is a deficient number, since it is larger than the sum of its proper divisors (10781).

21745891 is an equidigital number, since it uses as much as digits as its factorization.

21745891 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10780.

The product of its digits is 20160, while the sum is 37.

The square root of 21745891 is about 4663.2489746956. The cubic root of 21745891 is about 279.1209292363.

The spelling of 21745891 in words is "twenty-one million, seven hundred forty-five thousand, eight hundred ninety-one".