21128391 has 16 divisors (see below), whose sum is σ = 32312320. Its totient is φ = 13630680.

The previous prime is 21128381. The next prime is 21128411. The reversal of 21128391 is 19382112.

It is not a de Polignac number, because 21128391 - 2¹⁰ = 21127367 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21128321) 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 15 ways as a sum of consecutive naturals, for example, 11785 + ... + 13458.

It is an arithmetic number, because the mean of its divisors is an integer number (2019520).

Almost surely, 2^21128391 is an apocalyptic number.

21128391 is a deficient number, since it is larger than the sum of its proper divisors (11183929).

21128391 is a wasteful number, since it uses less digits than its factorization.

21128391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 25283 (or 25277 counting only the distinct ones).

The product of its digits is 864, while the sum is 27.

The square root of 21128391 is about 4596.5629550785. The cubic root of 21128391 is about 276.4535312736.

The spelling of 21128391 in words is "twenty-one million, one hundred twenty-eight thousand, three hundred ninety-one".