# Make mathematica treat $e_i^2$ as numeric

With NumericQ[symbol] = True, I can declare that a symbol is numeric. I want the expressions matching: $$e_{\text{i\_}?\text{IntegerQ}}^2$$ to be treated as numerical expressions too.

e /: NumericQ[Subscript[e, i_?IntegerQ]^2] = True;


doesn't work. Also, I can't use NumericFunction because it's too restrictive. Is there something like NumericPattern?

-

Here is another way: you can fool the depth-1 tag rule for UpValues with a few temporary symbols. Here is an example:

ClearAll[e];
e /: Subscript[e, i_?IntegerQ] := e /: Subscript[e, i] =
Module[{el},
el /: el^p_ := el /: el^p =
Module[{elp},
elp /: NumericQ[elp] = True;
elp
];
el /: NumericQ[el] = True;
el]


What this does is to substitute Subscript[e, i_?IntegerQ] by some symbol, which will print just as the original one, but will have some rules attached which will do what you need. Now,

NumericQ[Subscript[e,1]]
(* True *)

NumericQ[Subscript[e,1]^2]

(* True *)


The advantage of this method is that it is flexible. You are not tied to just powers of your subscript, it can be easily generalized to other functions.

-

Everything depends on what you try to do with this. First you could use Subsuperscript and get rid of one level which is introduced by your Power

Subscript[e,3]^2//FullForm
(* Power[Subscript[e,3],2] *)

Subsuperscript[e,3,2]//FullForm
(* Subsuperscript[e,3,2] *)


Both forms look equally in the front-end, but now you can use TagSet to transform all e with integer-subscript to a different form which displays as it would still be the normal e:

e/:Subsuperscript[e,i_?IntegerQ,2]:=eNumeric[i,2];
e/:Subsuperscript[e,i_,2]:=eNonNumeric[i,2];
SetAttributes[eNumeric,{NumericFunction}];

You can use Notation package to treat anything like function and then set whatever attributes to this function:
The only problem is that Notation don't support test patterns, so to make an expression numeric with only integer indexes:
@belisarius I didn't make it textual because those boxes inside Notation are very messy and unreadable. –  swish Dec 27 '12 at 13:46