There is a very typical math series that I want to type in Writer’s formula. It is as follows:

To do this, I write X_1, X_2, …,X_n, which results in the the following result:

I also, tried ““X_1,X_2,…,X_n””, `X_1,X_2,…,X_n`, and {X_1,X_2,…,X_n}, none of which worked. How should I type that?