system - Recursion problem

I try to make designation

a = {Subscript[a,1], Subscript[a,2], Subscript[a,3]}

but receive mistake

$RecursionLimit::reclim: Recursion depth of 1024 exceeded.

What`s the problem?

Answer

The reason is, that while assigning, the a in Subscript[a,_] is replaced by the whole list of subscripts, and so on and so on, until the recursion limit is reached: Subscripts are no symbols by themselves.

To avoid this, you can either use another variable to assign to (b e.g.), or define the subscripts as proper symbols:

Needs["Notation`"]

makesymbol[obj_]:=With[{},      
  If[NameQ@ToString@Unevaluated@obj,Remove@obj]; (* remove possibly existing symbol first *)
  Symbolize@ParsedBoxWrapper@ToBoxes@obj;]       (* then create the new symbol *)

With this function, you can create a symbol like so:

makesymbol[Subscript[a,1]]

Mathematica will now treat it the same as any other (simpler) symbol.

For your case:

Be sure to put the formatted variables into the list here, not the Subscript-form:

makesymbol /@ {Subscript[a, 1], Subscript[a, 2], Subscript[a, 3]}

and then your assignment will work without problems.