81949991 has 4 divisors (see below), whose sum is σ = 82362000. Its totient is φ = 81537984.

The previous prime is 81949981. The next prime is 81950003. The reversal of 81949991 is 19994918.

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 81949991 - 2¹⁸ = 81687847 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 (81949961) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 205706 + ... + 206103.

It is an arithmetic number, because the mean of its divisors is an integer number (20590500).

Almost surely, 2^81949991 is an apocalyptic number.

81949991 is a deficient number, since it is larger than the sum of its proper divisors (412009).

81949991 is a wasteful number, since it uses less digits than its factorization.

81949991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 412008.

The product of its digits is 209952, while the sum is 50.

The square root of 81949991 is about 9052.6234319119. The cubic root of 81949991 is about 434.3598122530.

The spelling of 81949991 in words is "eighty-one million, nine hundred forty-nine thousand, nine hundred ninety-one".