99991981 has 2 divisors, whose sum is σ = 99991982. Its totient is φ = 99991980.

The previous prime is 99991951. The next prime is 99991987. The reversal of 99991981 is 18919999.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 59923081 + 40068900 = 7741^2 + 6330^2 .

It is a cyclic number.

It is not a de Polignac number, because 99991981 - 2²¹ = 97894829 is a prime.

It is a super-2 number, since 2×99991981² = 19996792528608722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (99991987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 49995990 + 49995991.

It is an arithmetic number, because the mean of its divisors is an integer number (49995991).

Almost surely, 2^99991981 is an apocalyptic number.

It is an amenable number.

99991981 is a deficient number, since it is larger than the sum of its proper divisors (1).

99991981 is an equidigital number, since it uses as much as digits as its factorization.

99991981 is an evil number, because the sum of its binary digits is even.

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

The square root of 99991981 is about 9999.5990419616. The cubic root of 99991981 is about 464.1464760627.

It can be divided in two parts, 999919 and 81, that added together give a 6-th power (1000000 = 10⁶).

The spelling of 99991981 in words is "ninety-nine million, nine hundred ninety-one thousand, nine hundred eighty-one".