21138391 has 16 divisors (see below), whose sum is σ = 22462848. Its totient is φ = 19872000.

The previous prime is 21138379. The next prime is 21138401. The reversal of 21138391 is 19383112.

It is a cyclic number.

It is not a de Polignac number, because 21138391 - 2¹⁷ = 21007319 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21138371) 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 15 ways as a sum of consecutive naturals, for example, 209241 + ... + 209341.

It is an arithmetic number, because the mean of its divisors is an integer number (1403928).

Almost surely, 2^21138391 is an apocalyptic number.

21138391 is a deficient number, since it is larger than the sum of its proper divisors (1324457).

21138391 is a wasteful number, since it uses less digits than its factorization.

21138391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 282.

The product of its digits is 1296, while the sum is 28.

The square root of 21138391 is about 4597.6505956847. The cubic root of 21138391 is about 276.4971392523.

The spelling of 21138391 in words is "twenty-one million, one hundred thirty-eight thousand, three hundred ninety-one".