21015881 has 4 divisors (see below), whose sum is σ = 22122000. Its totient is φ = 19909764.

The previous prime is 21015871. The next prime is 21015913. The reversal of 21015881 is 18851012.

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 a cyclic number.

It is not a de Polignac number, because 21015881 - 2¹⁰ = 21014857 is a prime.

It is a super-2 number, since 2×21015881² = 883334508412322, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21015881.

It is not an unprimeable number, because it can be changed into a prime (21015851) 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, 553031 + ... + 553068.

It is an arithmetic number, because the mean of its divisors is an integer number (5530500).

Almost surely, 2^21015881 is an apocalyptic number.

It is an amenable number.

21015881 is a deficient number, since it is larger than the sum of its proper divisors (1106119).

21015881 is a wasteful number, since it uses less digits than its factorization.

21015881 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1106118.

The product of its (nonzero) digits is 640, while the sum is 26.

The square root of 21015881 is about 4584.3081266424. The cubic root of 21015881 is about 275.9619468996.

Adding to 21015881 its reverse (18851012), we get a palindrome (39866893).

The spelling of 21015881 in words is "twenty-one million, fifteen thousand, eight hundred eighty-one".