Re: Positive[a] = True ???
- Subject: [mg2618] Re: [mg2518] Positive[a] = True ???
- From: johan at kajsa.isy.liu.se (Johan Gunnarsson)
- Date: Thu, 30 Nov 1995 20:58:27 -0500
- Approved: usenet@wri.com
- Distribution: local
- Newsgroups: wri.mathgroup
- Organization: Wolfram Research, Inc.
If the symbol a is positive then why cannot Mathematica simplify
Sqrt[a^2] to a?
In[1]:= Positive[a]^=True;
In[2]:= Simplify[Sqrt[a^2]]
2
Out[2]= Sqrt[a ]
/Johan
In article <48ejb3$fi5 at ralph.vnet.net> Allan Hayes <hay at haystack.demon.co.uk> writes:
> Frank Domokos <domokos at chaph.usc.edu>
> in [mg2518] Positive[a] = True ???
> asks about the following kind of response
>
> IN>> Positive[a] = True;
> OUT>> Set::write: Tag Positive in Positive[a] is Protected.
>
> Frank,
>
> Most system functions, like Positive, are protected (have the
> attribute Protected) to avoid unintentional changes. You cannot make
> a definition for them ("tagged" by them) without unprotecting them
> (see later).
>
> Here are some ways round this for your example
>
> 1.Tag your definition with a instead of f:
>
> a. by using UpSet (^=) instead of Set (=):
>
> Positive[a]^= True;
>
> or, more specifically,
>
> b. by using TagSet:
>
> a/:Positive[a] = True;
>
> Or
>
> 2. Unprotect Positive to allow definitions to be tagged by it:
>
> Unprotect[Positive];
>
> Positive[a] = True;
>
> (eventually re-protecting Positive)
>
>
> Of course if a is protected or is an expression with a protected
> head then you will need to unprotect either Positive or a or the
> head of a.
>
> Allan Hayes
> hay at haystack.demon.co.uk
--
_____________________________________________________
Johan Gunnarsson
| Division of Automatic Control
/|\ Dept. of EE, Linkoping University
\|/ S-581 83 Linkoping, Sweden
/|\ Tel: +46 13 282913
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() | () Email: johan at isy.liu.se