20805 has 16 divisors (see below), whose sum is σ = 35520. Its totient is φ = 10368.

The previous prime is 20789. The next prime is 20807. The reversal of 20805 is 50802.

20805 = T₅ + T₆ + ... + T₄₉.

20805 is nontrivially palindromic in base 2, base 4 and base 8.

20805 is digitally balanced in base 6, because in such base it contains all the possibile digits an equal number of times.

It is not a de Polignac number, because 20805 - 2⁴ = 20789 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (15).

It is an Ulam number.

20805 is an undulating number in base 8.

20805 is strictly pandigital in base 6.

It is a Curzon number.

20805 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20807) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 249 + ... + 321.

It is an arithmetic number, because the mean of its divisors is an integer number (2220).

2²⁰⁸⁰⁵ is an apocalyptic number.

It is an amenable number.

20805 is a deficient number, since it is larger than the sum of its proper divisors (14715).

20805 is a wasteful number, since it uses less digits than its factorization.

20805 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 100.

The product of its (nonzero) digits is 80, while the sum is 15.

The square root of 20805 is about 144.2393843581. The cubic root of 20805 is about 27.5035808009.

Multiplying 20805 by its product of nonzero digits (80), we get a triangular number (1664400 = T₁₈₂₄).

The spelling of 20805 in words is "twenty thousand, eight hundred five".