20491 has 4 divisors (see below), whose sum is σ = 21184. Its totient is φ = 19800.

The previous prime is 20483. The next prime is 20507. The reversal of 20491 is 19402.

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

It is a cyclic number.

It is not a de Polignac number, because 20491 - 2³ = 20483 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 13 and base 15.

It is not an unprimeable number, because it can be changed into a prime (20411) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 300 + ... + 361.

It is an arithmetic number, because the mean of its divisors is an integer number (5296).

2²⁰⁴⁹¹ is an apocalyptic number.

20491 is a deficient number, since it is larger than the sum of its proper divisors (693).

20491 is an equidigital number, since it uses as much as digits as its factorization.

20491 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 692.

The product of its (nonzero) digits is 72, while the sum is 16.

The square root of 20491 is about 143.1467778191. The cubic root of 20491 is about 27.3645126387.

Adding to 20491 its reverse (19402), we get a palindrome (39893).

Subtracting from 20491 its reverse (19402), we obtain a square (1089 = 33²).

The spelling of 20491 in words is "twenty thousand, four hundred ninety-one".