999799981 has 2 divisors, whose sum is σ = 999799982. Its totient is φ = 999799980.

The previous prime is 999799979. The next prime is 999799987. The reversal of 999799981 is 189997999.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 535181956 + 464618025 = 23134^2 + 21555^2 .

It is an emirp because it is prime and its reverse (189997999) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 999799981 - 2¹ = 999799979 is a prime.

It is a super-2 number, since 2×999799981² = 1999200004015200722, which contains 22 as substring.

Together with 999799979, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 999799981.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (999799987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 499899990 + 499899991.

It is an arithmetic number, because the mean of its divisors is an integer number (499899991).

Almost surely, 2^999799981 is an apocalyptic number.

It is an amenable number.

999799981 is a deficient number, since it is larger than the sum of its proper divisors (1).

999799981 is an equidigital number, since it uses as much as digits as its factorization.

999799981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 29760696, while the sum is 70.

The square root of 999799981 is about 31619.6138654475. The cubic root of 999799981 is about 999.9333225542.

The spelling of 999799981 in words is "nine hundred ninety-nine million, seven hundred ninety-nine thousand, nine hundred eighty-one".