128517991 has 4 divisors (see below), whose sum is σ = 146877712. Its totient is φ = 110158272.

The previous prime is 128517973. The next prime is 128518003. The reversal of 128517991 is 199715821.

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

It is not a de Polignac number, because 128517991 - 2⁵ = 128517959 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 (128517901) 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, 9179850 + ... + 9179863.

It is an arithmetic number, because the mean of its divisors is an integer number (36719428).

Almost surely, 2^128517991 is an apocalyptic number.

128517991 is a deficient number, since it is larger than the sum of its proper divisors (18359721).

128517991 is an equidigital number, since it uses as much as digits as its factorization.

128517991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 18359720.

The product of its digits is 45360, while the sum is 43.

The square root of 128517991 is about 11336.5775699723. The cubic root of 128517991 is about 504.6473255685.

The spelling of 128517991 in words is "one hundred twenty-eight million, five hundred seventeen thousand, nine hundred ninety-one".