991900991 has 4 divisors (see below), whose sum is σ = 992012184. Its totient is φ = 991789800.

The previous prime is 991900939. The next prime is 991900993. The reversal of 991900991 is 199009199.

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 991900991 - 2¹⁰ = 991899967 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 (991900993) 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, 40925 + ... + 60486.

It is an arithmetic number, because the mean of its divisors is an integer number (248003046).

Almost surely, 2^991900991 is an apocalyptic number.

991900991 is a deficient number, since it is larger than the sum of its proper divisors (111193).

991900991 is a wasteful number, since it uses less digits than its factorization.

991900991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 111192.

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

The square root of 991900991 is about 31494.4596873799. The cubic root of 991900991 is about 997.2930091459.

The spelling of 991900991 in words is "nine hundred ninety-one million, nine hundred thousand, nine hundred ninety-one".