999997991 has 8 divisors (see below), whose sum is σ = 1027547904. Its totient is φ = 972709560.

The previous prime is 999997967. The next prime is 999998003. The reversal of 999997991 is 199799999.

It is a happy number.

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 999997991 - 2¹⁴ = 999981607 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 (999997891) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 57605 + ... + 72926.

It is an arithmetic number, because the mean of its divisors is an integer number (128443488).

Almost surely, 2^999997991 is an apocalyptic number.

999997991 is a deficient number, since it is larger than the sum of its proper divisors (27549913).

999997991 is a wasteful number, since it uses less digits than its factorization.

999997991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 130741.

The product of its digits is 33480783, while the sum is 71.

The square root of 999997991 is about 31622.7448365887. The cubic root of 999997991 is about 999.9993303329. Note that the first 3 decimals are identical.

The spelling of 999997991 in words is "nine hundred ninety-nine million, nine hundred ninety-seven thousand, nine hundred ninety-one".