999999981 has 6 divisors (see below), whose sum is σ = 1444444430. Its totient is φ = 666666648.

The previous prime is 999999937. The next prime is 1000000007. The reversal of 999999981 is 189999999.

It is a happy number.

It can be written as a sum of positive squares in only one way, i.e., 897302025 + 102697956 = 29955^2 + 10134^2 .

It is not a de Polignac number, because 999999981 - 2¹⁰ = 999998957 is a prime.

It is a super-2 number, since 2×999999981² = 1999999924000000722, which contains 22 as substring.

It is a Duffinian number.

It is a nialpdrome in base 10.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (999999181) 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 5 ways as a sum of consecutive naturals, for example, 55555546 + ... + 55555563.

Almost surely, 2^999999981 is an apocalyptic number.

It is an amenable number.

999999981 is a deficient number, since it is larger than the sum of its proper divisors (444444449).

999999981 is a wasteful number, since it uses less digits than its factorization.

999999981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 111111115 (or 111111112 counting only the distinct ones).

The product of its digits is 38263752, while the sum is 72.

The square root of 999999981 is about 31622.7763012674. The cubic root of 999999981 is about 999.9999936667. Note that the first 5 decimals are identical.

It can be divided in two parts, 99999998 and 1, that added together give a palindrome (99999999).

The spelling of 999999981 in words is "nine hundred ninety-nine million, nine hundred ninety-nine thousand, nine hundred eighty-one".