[texhax] Putting a prime on \sum

[Philip G. Ratcliffe]

> I just got to this question so I don't know what the original 
> was called. 
> Knuth raised this very question in The TeXBook and answered 
> it (Exercise 
> 18.44).  The first answer was to use \mathop{{\sum}'}_... but 
> that would 
> have the slightly unfortunate side-effect of centering the 
> subscript under 
> the primed \sum, rather than under the \sum.  He then goes on 
> the discuss 
> a more complicated solution, which would not have that 
> side-effect, but 
> since the TeXBook is available online, I won't copy it.
> 
> The following seemed to work too: 
> \def\sumprime_#1{\mathop{\sum_{#1}}\nolimits'} $$\s$$ 
> $$\sumprime_{x\in A}$$ Perhaps someone well-versed in 
> TeXarcana can explain why in Knuth's 
> wolution, the prime comes out to the right while in mine, if 
> you eliminate 
> the \nolimits (that is, no limit switch), it comes out atop the 
> summation.  Be that as it may, it appeared to work for me, 
> without the 
> unfortunate displacement of the subscript.  It has the 
> disadvantage of 
> requiring a subscript.  I have also not considered the 
> problem of having 
> also a superscript.  I guess you could use \@ifnextchar, with some 
> complication, to look ahead.

This solves some (but not all) of the problems inherent to this task:

\usepackage{mathtools}

\def\scriptafterop#1#2{\mathop{\smash{\mathrlap{#2\nolimits#1}}\phantom{#2}}
}

$$\scriptafterop{'}\sum_{k=1}^{\infty}$$

Cheers,  Phil