21081 has 4 divisors (see below), whose sum is σ = 28112. Its totient is φ = 14052.

The previous prime is 21067. The next prime is 21089. The reversal of 21081 is 18012.

It is a happy number.

21081 is nontrivially palindromic in base 5.

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 21081 - 2⁶ = 21017 is a prime.

It is a super-2 number, since 2×21081² = 888817122, which contains 22 as substring.

It is a D-number.

It is a Duffinian number.

21081 is a modest number, since divided by 81 gives 21 as remainder.

It is a zygodrome in base 5.

It is not an unprimeable number, because it can be changed into a prime (21089) 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 3 ways as a sum of consecutive naturals, for example, 3511 + ... + 3516.

It is an arithmetic number, because the mean of its divisors is an integer number (7028).

2²¹⁰⁸¹ is an apocalyptic number.

It is an amenable number.

21081 is a deficient number, since it is larger than the sum of its proper divisors (7031).

21081 is an equidigital number, since it uses as much as digits as its factorization.

21081 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7030.

The product of its (nonzero) digits is 16, while the sum is 12.

The square root of 21081 is about 145.1929750367. The cubic root of 21081 is about 27.6246681369.

Adding to 21081 its reverse (18012), we get a palindrome (39093).

The spelling of 21081 in words is "twenty-one thousand, eighty-one".