21009881 has 4 divisors (see below), whose sum is σ = 21019152. Its totient is φ = 21000612.

The previous prime is 21009871. The next prime is 21009883. The reversal of 21009881 is 18890012.

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, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21009881 - 2¹⁰ = 21008857 is a prime.

It is a super-2 number, since 2×21009881² = 882830199268322, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21009883) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1286 + ... + 6608.

It is an arithmetic number, because the mean of its divisors is an integer number (5254788).

Almost surely, 2^21009881 is an apocalyptic number.

It is an amenable number.

21009881 is a deficient number, since it is larger than the sum of its proper divisors (9271).

21009881 is an equidigital number, since it uses as much as digits as its factorization.

21009881 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9270.

The product of its (nonzero) digits is 1152, while the sum is 29.

The square root of 21009881 is about 4583.6536736538. The cubic root of 21009881 is about 275.9356821703.

Adding to 21009881 its reverse (18890012), we get a palindrome (39899893).

The spelling of 21009881 in words is "twenty-one million, nine thousand, eight hundred eighty-one".