91999981 has 2 divisors, whose sum is σ = 91999982. Its totient is φ = 91999980.

The previous prime is 91999979. The next prime is 91999993. The reversal of 91999981 is 18999919.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 78057225 + 13942756 = 8835^2 + 3734^2 .

It is an emirp because it is prime and its reverse (18999919) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 91999981 - 2¹ = 91999979 is a prime.

It is a super-2 number, since 2×91999981² = 16927993008000722, which contains 22 as substring.

Together with 91999979, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (91999951) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 45999990 + 45999991.

It is an arithmetic number, because the mean of its divisors is an integer number (45999991).

Almost surely, 2^91999981 is an apocalyptic number.

It is an amenable number.

91999981 is a deficient number, since it is larger than the sum of its proper divisors (1).

91999981 is an equidigital number, since it uses as much as digits as its factorization.

91999981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 472392, while the sum is 55.

The square root of 91999981 is about 9591.6620561819. The cubic root of 91999981 is about 451.4357124703.

The spelling of 91999981 in words is "ninety-one million, nine hundred ninety-nine thousand, nine hundred eighty-one".