Mathematical symbol to reference the i-th item in a tuple?

I figured I would collect a number of the comments together into an answer so you would have something to accept (citing, so no one would hate on me for an plagiarism).

As with many types of mathematical notations, there are a number of possible variations here.

• Sometimes $p_2(e)$ or $\pi_2(e)$ is used to denote the 2^nd projection (Martin Sleziak, FrancescoTurco).
• If you define an n-tuple as $\mathbf{x}\in\mathbb{R}^n$ (note the bold font), then the $i$th element can easily addressed with $x_i$ (Hauke Strasdat).
• Sometimes even $e^{(2)}$ or $e^2$ (lhf).

Just be sure you explain to the reader what you mean by the notation—don't assume he will understand (GEdgar).