9991991 has 4 divisors (see below), whose sum is σ = 9998352. Its totient is φ = 9985632.

The previous prime is 9991973. The next prime is 9991997. The reversal of 9991991 is 1991999.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-9991991 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, 1070 + ... + 4596.

It is an arithmetic number, because the mean of its divisors is an integer number (2499588).

Almost surely, 2^9991991 is an apocalyptic number.

9991991 is a deficient number, since it is larger than the sum of its proper divisors (6361).

9991991 is a wasteful number, since it uses less digits than its factorization.

9991991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6360.

The product of its digits is 59049, while the sum is 47.

The square root of 9991991 is about 3161.0110724260. The cubic root of 9991991 is about 215.3859374167.

The spelling of 9991991 in words is "nine million, nine hundred ninety-one thousand, nine hundred ninety-one".