81318781 has 2 divisors, whose sum is σ = 81318782. Its totient is φ = 81318780.

The previous prime is 81318779. The next prime is 81318817. The reversal of 81318781 is 18781318.

Together with next prime (81318817) it forms an Ormiston pair, because they use the same digits, order apart.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 69305625 + 12013156 = 8325^2 + 3466^2 .

It is a cyclic number.

It is not a de Polignac number, because 81318781 - 2¹ = 81318779 is a prime.

It is a super-2 number, since 2×81318781² = 13225488286651922, which contains 22 as substring.

Together with 81318779, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 81318781.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81388781) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40659390 + 40659391.

It is an arithmetic number, because the mean of its divisors is an integer number (40659391).

Almost surely, 2^81318781 is an apocalyptic number.

It is an amenable number.

81318781 is a deficient number, since it is larger than the sum of its proper divisors (1).

81318781 is an equidigital number, since it uses as much as digits as its factorization.

81318781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 10752, while the sum is 37.

The square root of 81318781 is about 9017.6926649781. The cubic root of 81318781 is about 433.2417352087.

The spelling of 81318781 in words is "eighty-one million, three hundred eighteen thousand, seven hundred eighty-one".