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If I have already used $i,j,k$ as indices, and need two more, where should I look? $l,m$?, $m$? I know from my previous questions that I can use anything I want as long as I define it, but I use $n$ quite a bit, and it seems that using $m$ as an index could be confusing.

I have thought of using $s$ and $t$, or $s_i$ and $t_i$ which are the initials of the categorical variables over which I am indexing? I don't think that there would be a confusion between the use of $s$ in $\beta{_\text{style}}_s$ and the standard use of $s$ for standard deviation... but perhaps there is a more standard convention that will be more clear / intuitive?

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    What would you use $m$ for, except for indexes and natural numbers? $s$ and $t$ tend to be used for continuous variables. If there are multiple indexes (and they form a reasonable sequence), you would usually use double index, like so: $x_{i_1},x_{i_2}\ldots,x_{i_n}$. I've seen as many as four indexes one on top over another (they were Greek characters, before you ask) in some way. At that point it does get quite confusing, but... What is intuitive really depends on the context, and how the various indexes relate to one another.2012-07-11
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    $a_l$ sometimes looks dangerously similar to $a_1$, so $a_\ell$ is preferable (as in the answer by @AsafKaraglia). (Of course, MSE/MathJax typesetting is another story altogether...)2012-07-11
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    I agree with Leonid. Please, never use $l$ in mathematical texts. Only use $\ell$.2012-07-11
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    If your indices all serve related purposes, maybe just $i_1, ... i_5$?2012-12-31

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