2
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Update: the following code proposed by Lukas works perfectly. I have tested both fraction or $a/b$ form, both work. I have revised the code a little bit, to make it more flexible:

  MakeBoxes[Sin[Rational[1, 2] Subscript[\[Alpha]_, i_]], 
  StandardForm] := 
 MakeBoxes[Subscript["s", Subscript[Overscript[\[Alpha], _], i]], 
 StandardForm]
MakeBoxes[Cos[Rational[1, 2] Subscript[\[Alpha]_, i_]], 
  StandardForm] := 
 MakeBoxes[Subscript["c", Subscript[Overscript[\[Alpha], _], i]], 
  StandardForm]

I learnt that we can use Makeboxes to display Sin[\alpha_i] as $s_{\alpha_i}$ as follows:

  (*MakeBoxes[Sin[Subscript[\[Alpha],i_]],StandardForm]:=
 MakeBoxes[Subscript[Subscript["s",\[Alpha]],i],StandardForm]

but can't find a way to make Sin[Subscript[[Alpha],i]/2] display as $s_{\bar{\alpha}_i}$, regardless of in fraction form or in $a/b$ form. I have been looking for this for quite a long time, but with no luck. I checked the documentation, but can't understand how Makeboxes works, especially in this context.

Could someone help to produce such a way so that it can take effect on both calculations involving fraction form or $a/b$ form? It'd be even better if some explanation on how to do it is provided.

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The issue you are facing is that the FullForm of x/2 changes upon evaluation, combined with the HoldAllComplete attribute of MakeBoxes:

Hold[a/2]//FullForm
(* Hold[Times[a,Power[2,-1]]] *)

a/2//FullForm
(* Times[Rational[1,2],a] *)

Note how $a\cdot2^{-1}$ changes to $a\cdot\frac 12$ when allowed to evaluate. As mentioned above, this leads to problems due to the HoldAllComplete attribute of MakeBoxes: You are defining a rule for the form you are inputting, which means it will not be applied to the evaluated form:

MakeBoxes[Sin[Subscript[α, i] / 2], StandardForm] =
  MakeBoxes[Subscript[s, Subscript[Overscript[α, _], 2]], StandardForm]

Sin[Subscript[\[Alpha], i]/2]

$\sin(\frac\alpha2)$

HoldForm[Sin[Subscript[\[Alpha], i]/2]]

$s_{\bar{\alpha}_2}$

To fix this, you need to define a formatting rule for the evaluated form:

MakeBoxes[Sin[Rational[1, 2] Subscript[α, i]], StandardForm] = 
  MakeBoxes[Subscript[s, Subscript[Overscript[α, _], 2]], StandardForm]

Sin[Subscript[\[Alpha], i]/2]

$s_{\bar{\alpha}_2}$

Alternatively, you can also inject the evaluated form into MakeBoxes:

With[
  {evaluated = Sin[Subscript[α, i] / 2]},
  MakeBoxes[evaluated, StandardForm] =
   MakeBoxes[Subscript[s, Subscript[Overscript[α, _], 2]], StandardForm]
]

Sin[Subscript[\[Alpha], i]/2]

$s_{\bar{\alpha}_2}$

Note that using Evaluate would not work, since HoldAllComplete prevents any and all forms of evaluation, including Evaluate and up-values.

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  • $\begingroup$ Thanks for your comment, which helps me solve the problem successfully. Notice that I made a typo in the subscript, I wanted it to be "i". I have revised it, and revised your code a little bit, to make it more flexible. Thanks! $\endgroup$ – larry Dec 7 '19 at 15:15

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