Question about formatting subscripts and superscripts for display

Question

I want to display a variable with multiple subscripts (divided by commas) and a superscript surrounded by parentheses.

Namely, I want something like this:

rulesForDisplaying = { a[n_,i_,k_] -> _____________ }

where the _____ is something that will make a[n,i,k]//rulesForDisplaying show up as

$$ a_{n,i}^{(k)} \ \ , $$

where $n$,$i$, and $k$ can be anything.

I've tried doing something like Superscript[Subscript[a,n,i],k], but there's two issues: (1) I don't know how to get artifical parentheses around k and (2) the k ends up displayed far off to the right as opposed to on top of the "n,i".

I apologize if this question has been asked before, but I haven't been able to find the relevant post. I'm using Mathematica 10.1, if that's relevant.

Thanks!

Answer 1

rulesForDisplaying[x_, n_, j_, z_] := 
\!\(\*SubsuperscriptBox[\(x\), \(n, 
  j\), \("\<(\>" <> ToString[z] <> "\<)\>"\)]\)

rulesForDisplaying[x, n, j, z]

$x_{n,j}^{\text{(z)}}$

or....

rulesForDisplaying2[a[n_, j_, z_]] := 
\!\(\*SubsuperscriptBox[\(a\), \(n, 
  j\), \("\<(\>" <> ToString[z] <> "\<)\>"\)]\)

rulesForDisplaying2[a[n, j, z]]

$a_{n,j}^{\text{(z)}}$

Answer 2

Format automatically formats for display

Format[a[n_, i_, k_]] :=
 Subsuperscript[a,
  ToString[n] <> "," <> ToString[i],
  "(" <> ToString[k] <> ")"]

{a[n, i, j], a[1, 2, 3]}