20391 has 8 divisors (see below), whose sum is σ = 31104. Its totient is φ = 11640.

The previous prime is 20389. The next prime is 20393. The reversal of 20391 is 19302.

It is an interprime number because it is at equal distance from previous prime (20389) and next prime (20393).

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 20391 - 2¹ = 20389 is a prime.

It is a nialpdrome in base 12.

It is a self number, because there is not a number n which added to its sum of digits gives 20391.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20393) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 465 + ... + 506.

It is an arithmetic number, because the mean of its divisors is an integer number (3888).

2²⁰³⁹¹ is an apocalyptic number.

20391 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

20391 is a deficient number, since it is larger than the sum of its proper divisors (10713).

20391 is an equidigital number, since it uses as much as digits as its factorization.

20391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 981.

The product of its (nonzero) digits is 54, while the sum is 15.

The square root of 20391 is about 142.7970587932. The cubic root of 20391 is about 27.3199253428.

Adding to 20391 its reverse (19302), we get a palindrome (39693).

Subtracting from 20391 its reverse (19302), we obtain a square (1089 = 33²).

The spelling of 20391 in words is "twenty thousand, three hundred ninety-one".