20813 has 4 divisors (see below), whose sum is σ = 22428. Its totient is φ = 19200.

The previous prime is 20809. The next prime is 20849. The reversal of 20813 is 31802.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 31802 = 2 ⋅15901.

It can be written as a sum of positive squares in 2 ways, for example, as 13924 + 6889 = 118^2 + 83^2 .

It is a cyclic number.

It is not a de Polignac number, because 20813 - 2² = 20809 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a nialpdrome in base 13.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20873) 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 3 ways as a sum of consecutive naturals, for example, 788 + ... + 813.

It is an arithmetic number, because the mean of its divisors is an integer number (5607).

2²⁰⁸¹³ is an apocalyptic number.

It is an amenable number.

20813 is a deficient number, since it is larger than the sum of its proper divisors (1615).

20813 is a wasteful number, since it uses less digits than its factorization.

20813 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1614.

The product of its (nonzero) digits is 48, while the sum is 14.

The square root of 20813 is about 144.2671133696. The cubic root of 20813 is about 27.5071056019.

It can be divided in two parts, 208 and 13, that multiplied together give a square (2704 = 52²).

The spelling of 20813 in words is "twenty thousand, eight hundred thirteen".