What do the three dots mean in this problem? (Concrete Mathematics, ex. 1.9)

Question

Exercise 1.9 in Concrete Mathematics (Knuth et al.):

What does '...' between $x_1$ and $x_n$ mean? If it's multiplication, I would expect the dots to be centered. If it's saying that the inequality holds for each $x_a$ with a between $1$ and $n$ (inclusive), then I would expect commas.