81720816399 has 18 divisors (see below), whose sum is σ = 125393229000. Its totient is φ = 51275804736.

The previous prime is 81720816391. The next prime is 81720816403. The reversal of 81720816399 is 99361802718.

81720816399 is a `hidden beast` number, since 8 + 172 + 0 + 81 + 6 + 399 = 666.

It is not a de Polignac number, because 81720816399 - 2³ = 81720816391 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81720816391) by changing a digit.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 15706899 + ... + 15712100.

It is an arithmetic number, because the mean of its divisors is an integer number (6966290500).

Almost surely, 2^81720816399 is an apocalyptic number.

81720816399 is a deficient number, since it is larger than the sum of its proper divisors (43672412601).

81720816399 is a wasteful number, since it uses less digits than its factorization.

81720816399 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 31419039 (or 31419019 counting only the distinct ones).

The product of its (nonzero) digits is 1306368, while the sum is 54.

The spelling of 81720816399 in words is "eighty-one billion, seven hundred twenty million, eight hundred sixteen thousand, three hundred ninety-nine".