20691 has 18 divisors (see below), whose sum is σ = 34580. Its totient is φ = 11880.

The previous prime is 20681. The next prime is 20693. The reversal of 20691 is 19602.

20691 = 18³ + 19³ + 20³.

It is not a de Polignac number, because 20691 - 2⁶ = 20627 is a prime.

It is a plaindrome in base 14.

It is a nialpdrome in base 12.

It is not an unprimeable number, because it can be changed into a prime (20693) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 1080 + ... + 1098.

2²⁰⁶⁹¹ is an apocalyptic number.

20691 is a deficient number, since it is larger than the sum of its proper divisors (13889).

20691 is a wasteful number, since it uses less digits than its factorization.

20691 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 47 (or 33 counting only the distinct ones).

The product of its (nonzero) digits is 108, while the sum is 18.

The square root of 20691 is about 143.8436651368. The cubic root of 20691 is about 27.4532539167.

Subtracting from 20691 its reverse (19602), we obtain a square (1089 = 33²).

The spelling of 20691 in words is "twenty thousand, six hundred ninety-one".