# How to type the mathematic series x_1,x_2,...,x_3 in Writer formula?

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?