20905 has 8 divisors (see below), whose sum is σ = 25992. Its totient is φ = 16128.

The previous prime is 20903. The next prime is 20921. The reversal of 20905 is 50902.

20905 is an esthetic number in base 12, because in such base its adjacent digits differ by 1.

It can be written as a sum of positive squares in 4 ways, for example, as 1024 + 19881 = 32^2 + 141^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 20905 - 2¹ = 20903 is a prime.

It is a Duffinian number.

20905 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (20903) 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 7 ways as a sum of consecutive naturals, for example, 129 + ... + 241.

It is an arithmetic number, because the mean of its divisors is an integer number (3249).

2²⁰⁹⁰⁵ is an apocalyptic number.

It is an amenable number.

20905 is a deficient number, since it is larger than the sum of its proper divisors (5087).

20905 is a wasteful number, since it uses less digits than its factorization.

20905 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 155.

The product of its (nonzero) digits is 90, while the sum is 16.

The square root of 20905 is about 144.5856147755. The cubic root of 20905 is about 27.5475760464.

The spelling of 20905 in words is "twenty thousand, nine hundred five".