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Topic: clearing a variable with a dummy subscript
Replies: 5   Last Post: May 21, 2012 5:59 AM

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Albert Retey

Posts: 688
Registered: 7/15/08
Re: clearing a variable with a dummy subscript
Posted: May 19, 2012 5:44 AM
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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




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