21508265781 has 12 divisors (see below), whose sum is σ = 31576799020. Its totient is φ = 14103780480.

The previous prime is 21508265759. The next prime is 21508265833. The reversal of 21508265781 is 18756280512.

21508265781 is a `hidden beast` number, since 2 + 1 + 50 + 8 + 26 + 578 + 1 = 666.

It can be written as a sum of positive squares in 2 ways, for example, as 3138800625 + 18369465156 = 56025^2 + 135534^2 .

It is not a de Polignac number, because 21508265781 - 2²³ = 21499877173 is a prime.

It is a super-2 number, since 2×21508265781² (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 (21508265741) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 19588036 + ... + 19589133.

Almost surely, 2^21508265781 is an apocalyptic number.

It is an amenable number.

21508265781 is a deficient number, since it is larger than the sum of its proper divisors (10068533239).

21508265781 is a wasteful number, since it uses less digits than its factorization.

21508265781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 39177236 (or 39177233 counting only the distinct ones).

The product of its (nonzero) digits is 268800, while the sum is 45.

The spelling of 21508265781 in words is "twenty-one billion, five hundred eight million, two hundred sixty-five thousand, seven hundred eighty-one".