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

Given a tuple e=(x,y), how do I reference the 2nd item (y)?

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Sometimes $p_2(e)$ is used to denote the 2nd projection. en.wikipedia.org/wiki/Projection_%28mathematics%29 –  Martin Sleziak Apr 9 '11 at 10:34
Or just $e_2$ . –  lhf Apr 9 '11 at 10:54
But, when you want to use one of these, explain it! You cannot assume the reader knows it already. –  GEdgar Apr 9 '11 at 14:08
I have seen $\pi_2\left(e\right)$, too. –  Francesco Turco Apr 9 '11 at 19:26
Sometimes also $e^{(2)}$ and even $e^2$. –  lhf Apr 11 '11 at 10:45
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• Sometimes $p_2(e)$ or $\pi_2(e)$ is used to denote the 2nd 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).