21256645381 has 2 divisors, whose sum is σ = 21256645382. Its totient is φ = 21256645380.

The previous prime is 21256645357. The next prime is 21256645477. The reversal of 21256645381 is 18354665212.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 11926824100 + 9329821281 = 109210^2 + 96591^2 .

It is a cyclic number.

It is not a de Polignac number, because 21256645381 - 2⁷ = 21256645253 is a prime.

It is a super-2 number, since 2×21256645381² (a number of 21 digits) contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21256645481) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10628322690 + 10628322691.

It is an arithmetic number, because the mean of its divisors is an integer number (10628322691).

Almost surely, 2^21256645381 is an apocalyptic number.

It is an amenable number.

21256645381 is a deficient number, since it is larger than the sum of its proper divisors (1).

21256645381 is an equidigital number, since it uses as much as digits as its factorization.

21256645381 is an evil number, because the sum of its binary digits is even.

The product of its digits is 345600, while the sum is 43.

The spelling of 21256645381 in words is "twenty-one billion, two hundred fifty-six million, six hundred forty-five thousand, three hundred eighty-one".