91681981 has 2 divisors, whose sum is σ = 91681982. Its totient is φ = 91681980.

The previous prime is 91681957. The next prime is 91681991. The reversal of 91681981 is 18918619.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 63059481 + 28622500 = 7941^2 + 5350^2 .

It is an emirp because it is prime and its reverse (18918619) is a distict prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-91681981 is a prime.

It is a super-2 number, since 2×91681981² = 16811171280168722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (91681991) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 45840990 + 45840991.

It is an arithmetic number, because the mean of its divisors is an integer number (45840991).

Almost surely, 2^91681981 is an apocalyptic number.

It is an amenable number.

91681981 is a deficient number, since it is larger than the sum of its proper divisors (1).

91681981 is an equidigital number, since it uses as much as digits as its factorization.

91681981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 31104, while the sum is 43.

The square root of 91681981 is about 9575.0708091377. The cubic root of 91681981 is about 450.9149794748.

The spelling of 91681981 in words is "ninety-one million, six hundred eighty-one thousand, nine hundred eighty-one".