128191991 has 4 divisors (see below), whose sum is σ = 129997584. Its totient is φ = 126386400.

The previous prime is 128191969. The next prime is 128192011. The reversal of 128191991 is 199191821.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 199191821 = 941 ⋅211681.

It is a cyclic number.

It is not a de Polignac number, because 128191991 - 2¹⁸ = 127929847 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 (128191961) 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, 902690 + ... + 902831.

It is an arithmetic number, because the mean of its divisors is an integer number (32499396).

Almost surely, 2^128191991 is an apocalyptic number.

128191991 is a deficient number, since it is larger than the sum of its proper divisors (1805593).

128191991 is an equidigital number, since it uses as much as digits as its factorization.

128191991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1805592.

The product of its digits is 11664, while the sum is 41.

The square root of 128191991 is about 11322.1902033131. The cubic root of 128191991 is about 504.2202664807.

The spelling of 128191991 in words is "one hundred twenty-eight million, one hundred ninety-one thousand, nine hundred ninety-one".