991999 has 2 divisors, whose sum is σ = 992000. Its totient is φ = 991998.

The previous prime is 991987. The next prime is 992011. The reversal of 991999 is 999199.

It is a balanced prime because it is at equal distance from previous prime (991987) and next prime (992011).

It is an emirp because it is prime and its reverse (999199) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 991999 - 2⁷ = 991871 is a prime.

991999 is a modest number, since divided by 999 gives 991 as remainder.

991999 is a lucky number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (991909) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495999 + 496000.

It is an arithmetic number, because the mean of its divisors is an integer number (496000).

2⁹⁹¹⁹⁹⁹ is an apocalyptic number.

991999 is a deficient number, since it is larger than the sum of its proper divisors (1).

991999 is an equidigital number, since it uses as much as digits as its factorization.

991999 is an evil number, because the sum of its binary digits is even.

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

The square root of 991999 is about 995.9914658269. The cubic root of 991999 is about 99.7325855325.

The spelling of 991999 in words is "nine hundred ninety-one thousand, nine hundred ninety-nine".