20405 has 16 divisors (see below), whose sum is σ = 31104. Its totient is φ = 12480.

The previous prime is 20399. The next prime is 20407. The reversal of 20405 is 50402.

It is not a de Polignac number, because 20405 - 2⁴ = 20389 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (11).

It is a Duffinian number.

It is a nialpdrome in base 12.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20407) 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, 359 + ... + 411.

It is an arithmetic number, because the mean of its divisors is an integer number (1944).

2²⁰⁴⁰⁵ is an apocalyptic number.

It is an amenable number.

20405 is a deficient number, since it is larger than the sum of its proper divisors (10699).

20405 is a wasteful number, since it uses less digits than its factorization.

20405 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 76.

The product of its (nonzero) digits is 40, while the sum is 11.

The square root of 20405 is about 142.8460709995. The cubic root of 20405 is about 27.3261763270.

Adding to 20405 its reverse (50402), we get a palindrome (70807).

It can be divided in two parts, 20 and 405, that multiplied together give a square (8100 = 90²).

The spelling of 20405 in words is "twenty thousand, four hundred five".