91999997981 has 2 divisors, whose sum is σ = 91999997982. Its totient is φ = 91999997980.

The previous prime is 91999997959. The next prime is 91999997993. The reversal of 91999997981 is 18979999919.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 60828329956 + 31171668025 = 246634^2 + 176555^2 .

It is an emirp because it is prime and its reverse (18979999919) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 91999997981 - 2¹⁸ = 91999735837 is a prime.

It is a super-2 number, since 2×91999997981² (a number of 23 digits) contains 22 as substring.

It is a junction number, because it is equal to n+sod(n) for n = 91999997896 and 91999997905.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (91999997081) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 45999998990 + 45999998991.

It is an arithmetic number, because the mean of its divisors is an integer number (45999998991).

Almost surely, 2^91999997981 is an apocalyptic number.

It is an amenable number.

91999997981 is a deficient number, since it is larger than the sum of its proper divisors (1).

91999997981 is an equidigital number, since it uses as much as digits as its factorization.

91999997981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 267846264, while the sum is 80.

The spelling of 91999997981 in words is "ninety-one billion, nine hundred ninety-nine million, nine hundred ninety-seven thousand, nine hundred eighty-one".