custom notation - Can we use letter with a subscript as a variable in Mathematica?

I tried

Subscript[a, 0] = 1

(* 1 *)

Clear[Subscript[a, 0]]

       During evaluation of Clear::ssym: Subscript[a, 0] is not a symbol or a string. >>

Clear[a]
Subscript[a, 0]
(* 1 *)

Any idea?

Answer

Yes you can, with limitations.

You have at least three different ways to make an assignment to a subscripted symbol a₀ :

1. 
   

   make a rule for Subscript

   
   


2. 
   
   

   make a rule for a

   
   


3. 
   

   "symbolize" a₀ using the Notation package/palette

   
   

In each case below, when I write e.g. Subscript[a, 1] this can also be entered as a₁ by typing a then Ctrl+_ then 1.

When you write:

Subscript[a, 1] = "dog";

You make an assignment to Subscript:

DownValues[Subscript]

{HoldPattern[a₁] :> "dog"}

You make a rule for a by using TagSet:

a /: Subscript[a, 2] = "cat";

UpValues[a]

{HoldPattern[a₂] :> "cat"}

If you use the Notation palette you mess with underlying Box forms behind the scenes, allowing for assignment to OwnValues:

Each of these can be cleared with either Unset or TagUnset:

Subscript[a, 1] =.

a /: Subscript[a, 2] =.