128939991 has 4 divisors (see below), whose sum is σ = 171919992. Its totient is φ = 85959992.

The previous prime is 128939989. The next prime is 128939999. The reversal of 128939991 is 199939821.

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 128939991 - 2¹ = 128939989 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (128939999) 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, 21489996 + ... + 21490001.

It is an arithmetic number, because the mean of its divisors is an integer number (42979998).

Almost surely, 2^128939991 is an apocalyptic number.

128939991 is a deficient number, since it is larger than the sum of its proper divisors (42980001).

128939991 is an equidigital number, since it uses as much as digits as its factorization.

128939991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 42980000.

The product of its digits is 314928, while the sum is 51.

The square root of 128939991 is about 11355.1746353810. The cubic root of 128939991 is about 505.1990732313.

The spelling of 128939991 in words is "one hundred twenty-eight million, nine hundred thirty-nine thousand, nine hundred ninety-one".