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Re: clearing a variable with a dummy subscript

• To: mathgroup at smc.vnet.net
• Subject: [mg126555] Re: clearing a variable with a dummy subscript
• From: A Retey <awnl at gmx-topmail.de>
• Date: Sat, 19 May 2012 05:42:48 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <jp54mp$gos$1@smc.vnet.net>

Am 18.05.2012 11:27, schrieb Andre Hautot:
> Hi !
> Here is something very simple I don't understand :
>
> f[x_] := x; f[2]    (*A function*)
>   2
>
> Clear[f]; f[2]       (*The same perfectly cleared as expected*)
> f[2]
>
>
> Subscript[u, k_] := k; Subscript[u, 2]        (*u with subscript k_
> entered via the palette*)
>   2
>
> Clear[u]; Subscript[u, 2]          (*I found no way to clear u; a
> suggestion ?)
> 2
>

Use:

Subscript[u, k_] =.

or (if you don't have other definitions for Subscript you want to keep):

Clear[Subscript]

the deeper reason is that this rule is stored in the DownValues of 
Subscript:

Subscript[u, k_] := k;
DownValues[Subscript]

What you probably want to do is to store it as an UpValue to u instead 
of a DownValue of Subscript:

u /: Subscript[u, k_] := k

DownValues[Subscript]
UpValues[u]

now you can get rid of the definition as expected:

Clear[u]

Subscript[u,2]


hth,

albert