81616781 has 2 divisors, whose sum is σ = 81616782. Its totient is φ = 81616780.

The previous prime is 81616753. The next prime is 81616789. The reversal of 81616781 is 18761618.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 60996100 + 20620681 = 7810^2 + 4541^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81616781 is a prime.

It is a super-2 number, since 2×81616781² = 13322597881603922, which contains 22 as substring.

It is a Sophie Germain prime.

It is an alternating number because its digits alternate between even and odd.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81616789) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40808390 + 40808391.

It is an arithmetic number, because the mean of its divisors is an integer number (40808391).

Almost surely, 2^81616781 is an apocalyptic number.

It is an amenable number.

81616781 is a deficient number, since it is larger than the sum of its proper divisors (1).

81616781 is an equidigital number, since it uses as much as digits as its factorization.

81616781 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 16128, while the sum is 38.

The square root of 81616781 is about 9034.2006287219. The cubic root of 81616781 is about 433.7703078578.

The spelling of 81616781 in words is "eighty-one million, six hundred sixteen thousand, seven hundred eighty-one".