21128709 has 16 divisors (see below), whose sum is σ = 32882688. Its totient is φ = 11816112.

The previous prime is 21128693. The next prime is 21128719. The reversal of 21128709 is 90782112.

21128709 is nontrivially palindromic in base 4.

It is not a de Polignac number, because 21128709 - 2⁴ = 21128693 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21128719) 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, 9717 + ... + 11690.

It is an arithmetic number, because the mean of its divisors is an integer number (2055168).

Almost surely, 2^21128709 is an apocalyptic number.

It is an amenable number.

21128709 is a deficient number, since it is larger than the sum of its proper divisors (11753979).

21128709 is a wasteful number, since it uses less digits than its factorization.

21128709 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 21464.

The product of its (nonzero) digits is 2016, while the sum is 30.

The square root of 21128709 is about 4596.5975460116. The cubic root of 21128709 is about 276.4549182191.

It can be divided in two parts, 211 and 28709, that added together give a triangular number (28920 = T₂₄₀).

The spelling of 21128709 in words is "twenty-one million, one hundred twenty-eight thousand, seven hundred nine".