51261691981 has 2 divisors, whose sum is σ = 51261691982. Its totient is φ = 51261691980.

The previous prime is 51261691973. The next prime is 51261692017. The reversal of 51261691981 is 18919616215.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 47422030756 + 3839661225 = 217766^2 + 61965^2 .

It is a cyclic number.

It is not a de Polignac number, because 51261691981 - 2³ = 51261691973 is a prime.

It is a super-2 number, since 2×51261691981² (a number of 22 digits) contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (51261691951) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (23) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 25630845990 + 25630845991.

It is an arithmetic number, because the mean of its divisors is an integer number (25630845991).

Almost surely, 2^51261691981 is an apocalyptic number.

It is an amenable number.

51261691981 is a deficient number, since it is larger than the sum of its proper divisors (1).

51261691981 is an equidigital number, since it uses as much as digits as its factorization.

51261691981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 233280, while the sum is 49.

The spelling of 51261691981 in words is "fifty-one billion, two hundred sixty-one million, six hundred ninety-one thousand, nine hundred eighty-one".