21508806581 has 4 divisors (see below), whose sum is σ = 21721765164. Its totient is φ = 21295848000.

The previous prime is 21508806547. The next prime is 21508806617. The reversal of 21508806581 is 18560880512.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 20152357681 + 1356448900 = 141959^2 + 36830^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21508806581 is a prime.

It is a super-2 number, since 2×21508806581² (a number of 21 digits) contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21508806511) 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, 106479140 + ... + 106479341.

It is an arithmetic number, because the mean of its divisors is an integer number (5430441291).

Almost surely, 2^21508806581 is an apocalyptic number.

It is an amenable number.

21508806581 is a deficient number, since it is larger than the sum of its proper divisors (212958583).

21508806581 is a wasteful number, since it uses less digits than its factorization.

21508806581 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 212958582.

The product of its (nonzero) digits is 153600, while the sum is 44.

The spelling of 21508806581 in words is "twenty-one billion, five hundred eight million, eight hundred six thousand, five hundred eighty-one".