20781 has 6 divisors (see below), whose sum is σ = 30030. Its totient is φ = 13848.

The previous prime is 20773. The next prime is 20789. The reversal of 20781 is 18702.

It is an interprime number because it is at equal distance from previous prime (20773) and next prime (20789).

It can be written as a sum of positive squares in only one way, i.e., 19881 + 900 = 141^2 + 30^2 .

It is not a de Polignac number, because 20781 - 2³ = 20773 is a prime.

It is a super-2 number, since 2×20781² = 863699922, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20789) 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 5 ways as a sum of consecutive naturals, for example, 1146 + ... + 1163.

It is an arithmetic number, because the mean of its divisors is an integer number (5005).

2²⁰⁷⁸¹ is an apocalyptic number.

It is an amenable number.

20781 is a deficient number, since it is larger than the sum of its proper divisors (9249).

20781 is a wasteful number, since it uses less digits than its factorization.

20781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2315 (or 2312 counting only the distinct ones).

The product of its (nonzero) digits is 112, while the sum is 18.

The square root of 20781 is about 144.1561653208. The cubic root of 20781 is about 27.4930009736.

The spelling of 20781 in words is "twenty thousand, seven hundred eighty-one".