Is there a way of writing subscript in one's answer (or question)?

Question

Is there a way to write subscript on EL&U?

Answer 1

TL;DR

Yes. Use the <sub>…</sub> tags.

For example:

In the series x₁, x₂, x₃, ⋯ x_n, we see …

There are corresponding <sup>…</sup> tags.

And for certain, specialized uses, there are specific Unicode characters you might find useful¹.

¹ _{Such as the superscript numerals}

More Formatting Options

I have yet to find one source that completely describes all the markdown and HTML features StackExchange supports, but these four together seem to cover the bases:

1. Editing help: list of all the standard Markdown features available in "plain text" editing.
2. Canonical Markdown reference (external): note that SE's Markdown may deviate from this spec in order to tailor the syntax for its own purposes.
3. HTML tags supported by SE: These are HTML tags, over-and-above the formatting features offered by Markdown, which SE supports. The <sub> and <sup> tags mentioned above fall into this category.
4. Magic links supported in comments: These are handy shortcuts for use in comments which make it easier to say common things, like "questions about the site belong on [Meta]", which will automagically convert the bracketed text to a link to that specific stack's meta.

Then there are small details like ```-style Markdown code blocks, which I'm not sure are comprehensively collected anywhere. If anyone does know of a single, central, consolidated, comprehensive, and authoritative reference, add a comment and I'll update this answer.