20991 has 4 divisors (see below), whose sum is σ = 27992. Its totient is φ = 13992.

The previous prime is 20983. The next prime is 21001. The reversal of 20991 is 19902.

It is a happy number.

20991 is nontrivially palindromic in base 9.

It is a semiprime because it is the product of two primes.

It is not a de Polignac number, because 20991 - 2³ = 20983 is a prime.

It is a D-number.

It is a Duffinian number.

20991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20921) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3496 + ... + 3501.

It is an arithmetic number, because the mean of its divisors is an integer number (6998).

2²⁰⁹⁹¹ is an apocalyptic number.

20991 is a deficient number, since it is larger than the sum of its proper divisors (7001).

20991 is an equidigital number, since it uses as much as digits as its factorization.

20991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7000.

The product of its (nonzero) digits is 162, while the sum is 21.

The square root of 20991 is about 144.8827111839. The cubic root of 20991 is about 27.5852998804.

Adding to 20991 its sum of digits (21), we get a palindrome (21012).

Subtracting from 20991 its reverse (19902), we obtain a square (1089 = 33²).

It can be divided in two parts, 209 and 91, that added together give a triangular number (300 = T₂₄).

The spelling of 20991 in words is "twenty thousand, nine hundred ninety-one".