991991 has 16 divisors (see below), whose sum is σ = 1333248. Its totient is φ = 712800.

The previous prime is 991987. The next prime is 991999. The reversal of 991991 is 199199.

It is a happy number.

It is not a de Polignac number, because 991991 - 2² = 991987 is a prime.

It is a super-2 number, since 2×991991² = 1968092288162, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (991999) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 506 + ... + 1496.

It is an arithmetic number, because the mean of its divisors is an integer number (83328).

2⁹⁹¹⁹⁹¹ is an apocalyptic number.

991991 is a gapful number since it is divisible by the number (91) formed by its first and last digit.

991991 is a deficient number, since it is larger than the sum of its proper divisors (341257).

991991 is a wasteful number, since it uses less digits than its factorization.

991991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1022.

The product of its digits is 6561, while the sum is 38.

The square root of 991991 is about 995.9874497201. The cubic root of 991991 is about 99.7323174331.

The spelling of 991991 in words is "nine hundred ninety-one thousand, nine hundred ninety-one".