How to define a \bigplus operator in plain TeX?

Question

How to define a big plus operator that behaves exactly like bigsum but uses the plus symbol instead of a capital sigma?

I know that the best option (or at least the way Don would do it) is probably to design this symbol using metafont but scaling is good enough for me.

David Carlisle · Accepted Answer · 2021-10-08T00:37:11.520

4

You can use \mathchoice to use a suitably scaled font in each math style.


\font \zzz = cmr10 at 14pt
\font\zzzb  cmr10 at 20pt
\font\zzzsy =cmsy10 at 14pt
\font\zzzsyb =cmsy10 at 20pt
\def\bigplus{\mathop{\mathchoice{\hbox{\zzzb+}}{\hbox{\zzz+}}{\scriptstyle+}{\scriptscriptstyle+}}}
\def\bigtimes{\mathop{\mathchoice{\hbox{\zzzsyb\char2}}{\hbox{\zzzsy\char2}}{\scriptstyle\times}{\scriptscriptstyle\times}}}
\def\test{\sum_{i=0}^nx^i \rightarrow \bigplus_{i=0}^nx^i \rightarrow \bigtimes_{i=0}^nx^i}
$$
\test
$$
or..
\bigskip
This $\test$
\bigskip
Or this $x^{\test}$
\bye

Or with vertical center for Barbara

\def\bigplus{\mathop{\mathchoice{\vcenter{\hbox{\zzzb+}}}{\vcenter{\hbox{\zzz+}}}{\scriptstyle+}{\scriptscriptstyle+}}}
\def\bigtimes{\mathop{\mathchoice{\vcenter{\hbox{\zzzsyb\char2}}}{\vcenter{\hbox{\zzzsy\char2}}}{\scriptstyle\times}{\scriptscriptstyle\times}}}

edited Oct 08 '21 at 00:37

answered Oct 07 '21 at 20:19

David Carlisle

757,742

Thank you for your answer but I don't understand why the same font isn't used in the two examples and what's the meaning of zzz, thank you – Oct 07 '21 at 20:25
@Laravel + comes from the roman font and \times comes from the symbols font in the default plain tex layout. zzz is just a name you could call them \bigrm \biggerrm, \bigsym, \biggersym if you prefer. – David Carlisle Oct 07 '21 at 20:35
Thank you that makes sense! Were the 14pt and 20pt values chosen arbitrarily? – Oct 07 '21 at 20:40
yes I just did it by eye. @Laravel – David Carlisle Oct 07 '21 at 20:41
1

You really want to treat those enlarged symbols like "large operators' so that they're centered vertically on the math axis. (Please!) By the way, the selection of 20pt and 14pt are appropriate.]; that's what was used when this had to be done at AMS. – barbara beeton Oct 08 '21 at 00:27
@barbarabeeton I made them \mathop oh you want them vcentred as well? hmm I guess so let me try that... – David Carlisle Oct 08 '21 at 00:29
@barbarabeeton at your service.... – David Carlisle Oct 08 '21 at 00:37
Oh, that's much better! (I.e., correctly placed.) – barbara beeton Oct 08 '21 at 00:53
@barbarabeeton I even spelled centre incorrectly for you. – David Carlisle Oct 08 '21 at 00:54

score 1 · Answer 2 · answered Oct 08 '21 at 07:25

You can do it with scaling, provided you load eplain. The following code is based on https://tex.stackexchange.com/a/23436/4427. The idea is to measure \sum in the current math style and to scale the chosen symbol to match.

Experiment and choose the right factor for \genericbigop and the chosen symbol.

\input eplain
\beginpackages
\usepackage{graphicx}
\endpackages
% add scaled versions of the cmex font
\font\sevenex=cmex7
\font\fiveex=cmex7 at 5pt
\scriptfont3=\sevenex
\scriptscriptfont3=\fiveex
%
\catcode@=11 \def\genericbigop#1#2{% #1=factor, #2=symbol \mathop{\vphantom{\sum}\mathpalette\genericbigop@{{#1}{#2}}}\displaylimits } \def\genericbigop@#1#2{\genericbigop@@#1#2} \def\genericbigop@@#1#2#3{% \vcenter{% \setbox\z@=\hbox{$#1\sum$}% \dimen@=\ht\z@ \advance\dimen@\dp\z@ \hbox{\resizebox{#2\dimen@}{!}{$\m@th#3$}}% }% } \catcode@=12
\def\bigplus{\genericbigop{1.1}{+}}
\def\bigtimes{\genericbigop{1.1}{\times}}
$$
\sum_{i=0}^nx^i \quad \bigplus_{i=0}^nx^i \quad \bigtimes_{i=0}^nx^i
$$
\centerline{%
  $\sum_{i=0}^nx^i \quad \bigplus_{i=0}^nx^i \quad \bigtimes_{i=0}^nx^i$
}
\centerline{%
  $\scriptstyle\sum_{i=0}^nx^i \quad \bigplus_{i=0}^nx^i \quad \bigtimes_{i=0}^nx^i$
}
\centerline{%
  $\scriptscriptstyle\sum_{i=0}^nx^i \quad \bigplus_{i=0}^nx^i \quad \bigtimes_{i=0}^nx^i$
}
\bye

How to define a \bigplus operator in plain TeX?

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