81991909 has 4 divisors (see below), whose sum is σ = 82023520. Its totient is φ = 81960300.

The previous prime is 81991883. The next prime is 81991913. The reversal of 81991909 is 90919918.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 81991909 - 2⁹ = 81991397 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 (81991919) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 11529 + ... + 17230.

It is an arithmetic number, because the mean of its divisors is an integer number (20505880).

Almost surely, 2^81991909 is an apocalyptic number.

It is an amenable number.

81991909 is a deficient number, since it is larger than the sum of its proper divisors (31611).

81991909 is a wasteful number, since it uses less digits than its factorization.

81991909 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 31610.

The product of its (nonzero) digits is 52488, while the sum is 46.

The square root of 81991909 is about 9054.9383763778. The cubic root of 81991909 is about 434.4338590013.

The spelling of 81991909 in words is "eighty-one million, nine hundred ninety-one thousand, nine hundred nine".