21391 has 2 divisors, whose sum is σ = 21392. Its totient is φ = 21390.

The previous prime is 21383. The next prime is 21397. The reversal of 21391 is 19312.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21391 - 2³ = 21383 is a prime.

It is a nialpdrome in base 13 and base 15.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21397) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10695 + 10696.

It is an arithmetic number, because the mean of its divisors is an integer number (10696).

2²¹³⁹¹ is an apocalyptic number.

21391 is the 93-rd centered pentagonal number.

21391 is a deficient number, since it is larger than the sum of its proper divisors (1).

21391 is an equidigital number, since it uses as much as digits as its factorization.

21391 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 54, while the sum is 16.

The square root of 21391 is about 146.2566237816. The cubic root of 21391 is about 27.7594183868.

The spelling of 21391 in words is "twenty-one thousand, three hundred ninety-one".