21128891 has 4 divisors (see below), whose sum is σ = 24147312. Its totient is φ = 18110472.

The previous prime is 21128873. The next prime is 21128917. The reversal of 21128891 is 19882112.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21128891 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21128851) 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, 1509200 + ... + 1509213.

It is an arithmetic number, because the mean of its divisors is an integer number (6036828).

Almost surely, 2^21128891 is an apocalyptic number.

21128891 is a deficient number, since it is larger than the sum of its proper divisors (3018421).

21128891 is an equidigital number, since it uses as much as digits as its factorization.

21128891 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3018420.

The product of its digits is 2304, while the sum is 32.

The square root of 21128891 is about 4596.6173432210. The cubic root of 21128891 is about 276.4557119992.

The spelling of 21128891 in words is "twenty-one million, one hundred twenty-eight thousand, eight hundred ninety-one".