As I mentioned in my previous question,
a is a determiner used with a noun. However, I am confused about the usage of
a in the following sentences.
Let (x_1, x_2) be a pair of variables.
The second sentence is wrong. Why?
We fit a mixture of Gaussian models.
I think the first sentence is correct. That is because a refers to the word pair, not the variables. However, the second sentence is wrong. Someone told me that I should not use a as we have the word models which is plural. I am really confused why a in the second sentence refers to models and not to mixture.