21500891 has 4 divisors (see below), whose sum is σ = 21552384. Its totient is φ = 21449400.

The previous prime is 21500833. The next prime is 21500893. The reversal of 21500891 is 19800512.

It is a happy number.

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 21500891 - 2⁶ = 21500827 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 21500891.

It is not an unprimeable number, because it can be changed into a prime (21500893) 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, 25115 + ... + 25956.

It is an arithmetic number, because the mean of its divisors is an integer number (5388096).

Almost surely, 2^21500891 is an apocalyptic number.

21500891 is a deficient number, since it is larger than the sum of its proper divisors (51493).

21500891 is an equidigital number, since it uses as much as digits as its factorization.

21500891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 51492.

The product of its (nonzero) digits is 720, while the sum is 26.

The square root of 21500891 is about 4636.9053257534. The cubic root of 21500891 is about 278.0687294484.

The spelling of 21500891 in words is "twenty-one million, five hundred thousand, eight hundred ninety-one".