21256476781 has 4 divisors (see below), whose sum is σ = 21257567964. Its totient is φ = 21255385600.

The previous prime is 21256476701. The next prime is 21256476823. The reversal of 21256476781 is 18767465212.

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 8106121156 + 13150355625 = 90034^2 + 114675^2 .

It is a cyclic number.

It is not a de Polignac number, because 21256476781 - 2²⁹ = 20719605869 is a prime.

It is a super-2 number, since 2×21256476781² (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 (21256476701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 515830 + ... + 555511.

It is an arithmetic number, because the mean of its divisors is an integer number (5314391991).

Almost surely, 2^21256476781 is an apocalyptic number.

It is an amenable number.

21256476781 is a deficient number, since it is larger than the sum of its proper divisors (1091183).

21256476781 is a wasteful number, since it uses less digits than its factorization.

21256476781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1091182.

The product of its digits is 1128960, while the sum is 49.

The spelling of 21256476781 in words is "twenty-one billion, two hundred fifty-six million, four hundred seventy-six thousand, seven hundred eighty-one".