21231391 has 2 divisors, whose sum is σ = 21231392. Its totient is φ = 21231390.

The previous prime is 21231389. The next prime is 21231403. The reversal of 21231391 is 19313212.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21231391 - 2¹ = 21231389 is a prime.

Together with 21231389, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21234391) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10615695 + 10615696.

It is an arithmetic number, because the mean of its divisors is an integer number (10615696).

Almost surely, 2^21231391 is an apocalyptic number.

21231391 is a deficient number, since it is larger than the sum of its proper divisors (1).

21231391 is an equidigital number, since it uses as much as digits as its factorization.

21231391 is an evil number, because the sum of its binary digits is even.

The product of its digits is 324, while the sum is 22.

The square root of 21231391 is about 4607.7533571145. The cubic root of 21231391 is about 276.9020362832.

The spelling of 21231391 in words is "twenty-one million, two hundred thirty-one thousand, three hundred ninety-one".