999991 has 8 divisors (see below), whose sum is σ = 1077840. Its totient is φ = 924288.

The previous prime is 999983. The next prime is 1000003. The reversal of 999991 is 199999.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 999991 - 2³ = 999983 is a prime.

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 (999931) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 505 + ... + 1501.

It is an arithmetic number, because the mean of its divisors is an integer number (134730).

2⁹⁹⁹⁹⁹¹ is an apocalyptic number.

999991 is a deficient number, since it is larger than the sum of its proper divisors (77849).

999991 is a wasteful number, since it uses less digits than its factorization.

999991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1073.

The product of its digits is 59049, while the sum is 46.

The square root of 999991 is about 999.9954999899. The cubic root of 999991 is about 99.9996999991. Note that the first 3 decimals are identical.

It can be divided in two parts, 99999 and 1, that added together give a 5-th power (100000 = 10⁵).

The spelling of 999991 in words is "nine hundred ninety-nine thousand, nine hundred ninety-one".