128991 has 16 divisors (see below), whose sum is σ = 189440. Its totient is φ = 77760.

The previous prime is 128987. The next prime is 128993. The reversal of 128991 is 199821.

128991 is nontrivially palindromic in base 2.

It is not a de Polignac number, because 128991 - 2² = 128987 is a prime.

It is a Duffinian number.

128991 is an undulating number in base 8.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (128993) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 1731 + ... + 1803.

It is an arithmetic number, because the mean of its divisors is an integer number (11840).

2¹²⁸⁹⁹¹ is an apocalyptic number.

128991 is a deficient number, since it is larger than the sum of its proper divisors (60449).

128991 is a wasteful number, since it uses less digits than its factorization.

128991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 126.

The product of its digits is 1296, while the sum is 30.

The square root of 128991 is about 359.1531706668. The cubic root of 128991 is about 50.5265683810.

The spelling of 128991 in words is "one hundred twenty-eight thousand, nine hundred ninety-one".