991081981 has 12 divisors (see below), whose sum is σ = 1009713276. Its totient is φ = 972693120.

The previous prime is 991081979. The next prime is 991081991. The reversal of 991081981 is 189180199.

991081981 = T₂₅₇₀₃ + T₂₅₇₀₄ + T₂₅₇₀₅.

991081981 is a nontrivial binomial coefficient, being equal to C(44522, 2).

It can be written as a sum of positive squares in 2 ways, for example, as 490844025 + 500237956 = 22155^2 + 22366^2 .

It is not a de Polignac number, because 991081981 - 2¹ = 991081979 is a prime.

It is a super-2 number, since 2×991081981² = 1964486986125768722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (991081921) 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 in 11 ways as a sum of consecutive naturals, for example, 4696966 + ... + 4697176.

It is an arithmetic number, because the mean of its divisors is an integer number (84142773).

Almost surely, 2^991081981 is an apocalyptic number.

991081981 is the 44521-st triangular number and also the 22261-st hexagonal number.

991081981 is the 25705-th centered triangular number and also the 14841-st centered nonagonal number.

It is an amenable number.

991081981 is a deficient number, since it is larger than the sum of its proper divisors (18631295).

991081981 is a wasteful number, since it uses less digits than its factorization.

991081981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 732 (or 521 counting only the distinct ones).

The product of its (nonzero) digits is 46656, while the sum is 46.

The square root of 991081981 is about 31481.4545566116. The cubic root of 991081981 is about 997.0184461718.

The spelling of 991081981 in words is "nine hundred ninety-one million, eighty-one thousand, nine hundred eighty-one".