20581 has 4 divisors (see below), whose sum is σ = 22464. Its totient is φ = 18700.

The previous prime is 20563. The next prime is 20593. The reversal of 20581 is 18502.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 20581 - 2⁵ = 20549 is a prime.

It is a super-2 number, since 2×20581² = 847155122, which contains 22 as substring.

It is a Duffinian number.

20581 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20521) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 925 + ... + 946.

It is an arithmetic number, because the mean of its divisors is an integer number (5616).

2²⁰⁵⁸¹ is an apocalyptic number.

It is an amenable number.

20581 is a deficient number, since it is larger than the sum of its proper divisors (1883).

20581 is a wasteful number, since it uses less digits than its factorization.

20581 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1882.

The product of its (nonzero) digits is 80, while the sum is 16.

The square root of 20581 is about 143.4607960385. The cubic root of 20581 is about 27.4045173435.

The spelling of 20581 in words is "twenty thousand, five hundred eighty-one".