21881 has 2 divisors, whose sum is σ = 21882. Its totient is φ = 21880.

The previous prime is 21871. The next prime is 21893. The reversal of 21881 is 18812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 11881 + 10000 = 109^2 + 100^2 .

It is a cyclic number.

It is not a de Polignac number, because 21881 - 2⁶ = 21817 is a prime.

It is a super-2 number, since 2×21881² = 957556322, which contains 22 as substring.

It is a Chen prime.

It is a plaindrome in base 16.

It is not a weakly prime, because it can be changed into another prime (21821) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10940 + 10941.

It is an arithmetic number, because the mean of its divisors is an integer number (10941).

2²¹⁸⁸¹ is an apocalyptic number.

It is an amenable number.

21881 is a deficient number, since it is larger than the sum of its proper divisors (1).

21881 is an equidigital number, since it uses as much as digits as its factorization.

21881 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 128, while the sum is 20.

The square root of 21881 is about 147.9222768889. The cubic root of 21881 is about 27.9697803216.

The spelling of 21881 in words is "twenty-one thousand, eight hundred eighty-one".