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I have two $i^{th}$ and $i^{\prime th}$ and it is hard to see prime in $i^{\prime th}$. I would like to know the opinion of expert LaTeX users about this. I wonder what is the best and recommend way to write $i^{th}$ and $i^{\prime th}$.

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I agree with egreg's answer below, but if you must use a superscript, at least use $i^\text{th}. Not using \text or \mathrm for text in equations is among the most annoying Latex errors in the eye of this beholder. –  Nagel Jun 4 at 9:36

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up vote 13 down vote accepted

The superscript "th" seems to have come into use in the Victorian age; then it was dropped and it resurrected when some word processor made it their default. See the nice column on TUGboat by Peter Flynn about this (TUGboat 26, n. 3, 2005, p.~196).

In mathematical writing such a use can be confused with exponents: what does

$i^{th}$

mean? Is it "i raised to the power th" or an ordinal?

So I believe it is best to use

$i$-th

where the hyphen is clearly not a minus and so this construction is semantically sound. In case of the $i'$-th element one might think whether expanding it into the element at place~$i'$ is better.

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I don't think the hyphen is clearly not a minus any more than the superscript is an exponent. –  Emre Apr 27 '12 at 22:44
4  
@Emre The hyphen is usually much shorter than the minus sign; moreover the "i" is in italics, while "th" is roman (unless this is in an italics context, where the "expanded" construction might be better). –  egreg Apr 27 '12 at 22:51
    
@egreg: I prefer $i$-th too, but I have seen some claim that $i$th is more correct. Do you have any insight into which of these is more standard in mathematical writing? –  Nagel Jun 4 at 9:33
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@Nagel I've seen both forms. If several instances of the construction are used, defining a macro is in order, so one can change all of them in just one place. –  egreg Jun 4 at 9:37

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