81991423 has 4 divisors (see below), whose sum is σ = 82049080. Its totient is φ = 81933768.

The previous prime is 81991421. The next prime is 81991433. The reversal of 81991423 is 32419918.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 81991423 - 2¹ = 81991421 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81991421) 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 in 3 ways as a sum of consecutive naturals, for example, 26640 + ... + 29557.

It is an arithmetic number, because the mean of its divisors is an integer number (20512270).

Almost surely, 2^81991423 is an apocalyptic number.

81991423 is a deficient number, since it is larger than the sum of its proper divisors (57657).

81991423 is a wasteful number, since it uses less digits than its factorization.

81991423 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 57656.

The product of its digits is 15552, while the sum is 37.

The square root of 81991423 is about 9054.9115401532. The cubic root of 81991423 is about 434.4330006431.

The spelling of 81991423 in words is "eighty-one million, nine hundred ninety-one thousand, four hundred twenty-three".