81908993 has 4 divisors (see below), whose sum is σ = 83031108. Its totient is φ = 80786880.

The previous prime is 81908989. The next prime is 81909011. The reversal of 81908993 is 39980918.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 39980918 = 2 ⋅19990459.

It can be written as a sum of positive squares in 2 ways, for example, as 29953729 + 51955264 = 5473^2 + 7208^2 .

It is a cyclic number.

It is not a de Polignac number, because 81908993 - 2² = 81908989 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81908923) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 560948 + ... + 561093.

It is an arithmetic number, because the mean of its divisors is an integer number (20757777).

Almost surely, 2^81908993 is an apocalyptic number.

It is an amenable number.

81908993 is a deficient number, since it is larger than the sum of its proper divisors (1122115).

81908993 is a wasteful number, since it uses less digits than its factorization.

81908993 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1122114.

The product of its (nonzero) digits is 139968, while the sum is 47.

The square root of 81908993 is about 9050.3587221723. The cubic root of 81908993 is about 434.2873662249.

The spelling of 81908993 in words is "eighty-one million, nine hundred eight thousand, nine hundred ninety-three".