20891 has 4 divisors (see below), whose sum is σ = 22512. Its totient is φ = 19272.

The previous prime is 20887. The next prime is 20897. The reversal of 20891 is 19802.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19802 = 2 ⋅9901.

It is a cyclic number.

It is not a de Polignac number, because 20891 - 2² = 20887 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20897) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 791 + ... + 816.

It is an arithmetic number, because the mean of its divisors is an integer number (5628).

2²⁰⁸⁹¹ is an apocalyptic number.

20891 is a deficient number, since it is larger than the sum of its proper divisors (1621).

20891 is a wasteful number, since it uses less digits than its factorization.

20891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1620.

The product of its (nonzero) digits is 144, while the sum is 20.

The square root of 20891 is about 144.5371924454. The cubic root of 20891 is about 27.5414251703.

Subtracting from 20891 its reverse (19802), we obtain a square (1089 = 33²).

The spelling of 20891 in words is "twenty thousand, eight hundred ninety-one".