9991999 has 4 divisors (see below), whose sum is σ = 10118560. Its totient is φ = 9865440.

The previous prime is 9991997. The next prime is 9992009.

It is a happy number.

9991999 is nontrivially palindromic in base 10.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 9991999 - 2¹ = 9991997 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (9991997) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 63162 + ... + 63319.

It is an arithmetic number, because the mean of its divisors is an integer number (2529640).

Almost surely, 2^9991999 is an apocalyptic number.

9991999 is a deficient number, since it is larger than the sum of its proper divisors (126561).

9991999 is a wasteful number, since it uses less digits than its factorization.

9991999 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 126560.

The product of its digits is 531441, while the sum is 55.

The square root of 9991999 is about 3161.0123378437. The cubic root of 9991999 is about 215.3859948989.

It can be divided in two parts, 999 and 1999, that multiplied together give a triangular number (1997001 = T₁₉₉₈).

The spelling of 9991999 in words is "nine million, nine hundred ninety-one thousand, nine hundred ninety-nine".