TeXForm: control processing spelt-out names of Greek letters

Question

In a notebook, type in

theta^2

then select it and use Copy As → LaTeX. The resulting LaTeX code is

\text {theta}^2

Now try

TeXForm[theta^2]
(* \theta ^2 *)

Note that theta turned into \theta in the second case.

Why is there a difference between copying and using TeXForm directly? How can I control whether spelt-out names of greek letters, such as theta, will be converted into the actual Greek letter, i.e. \theta?

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One possible, but slightly inconvenient way to prevent the processing of such names is to replace them with strings, theta -> "theta". I do not usually mind this processing (I actually prefer it), I am simply looking for a way to control it.

Answer 1

It has something to do with these: Convert`TeX`BoxesToTeX[ToBoxes[theta^2]] and Convert`TeX`ExpressionToTeX[theta^2]. That and copying from a notebook deals with boxes and TeXForm deals with expressions. Convert`TeX`ExpressionToTeX calls Convert`TeX`BoxesToTeX with the option

"BoxRules" -> System`Convert`TeXFormDump`$GreekWords

but copying as TeX does not.

Managing TeXForm

TeXForm[expr] applies the list of rules System`Convert`TeXFormDump`$GreekWords when it is typeset.

Demonstration:

Block[{System`Convert`TeXFormDump`$GreekWords = {}},
 CellPrint@ExpressionCell[TeXForm[theta^2], "Output"]
 ]

Note that

Block[{System`Convert`TeXFormDump`$GreekWords = {}},
 TeXForm[theta^2]
 ]

does not work because the output is TeXForm[theta^2], which is then typeset after exiting the Block[]. One can set the value of $GreekWords globally, so that TeXForm[theta^2] would be typeset without converting words to letters.

Managing CopyAs > LaTeX

This is but a partial answer. Setting the following option should work to make CopyAs > LaTeX convert Greek letter names to letters, assuming System`Convert`TeXFormDump`$GreekWords retains its default value:

SetOptions[Convert`TeX`BoxesToTeX, 
  "BoxRules" -> System`Convert`TeXFormDump`$GreekWords];

On a Mac, it works when I use the context menu (CTRL-click, CopyAs > LaTeX). However, I find it does not work with the menu-bar command Edit > CopyAs > LaTeX. (I have no clue why as yet, nor how it works on other systems.)