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From: Peter Pein on 8 Aug 2010 07:22
Am Sat, 7 Aug 2010 10:22:42 +0000 (UTC)
schrieb "Christoph Lhotka" <christoph.lhotka(a)univie.ac.at>:

> Hello,
>
> Sorry for my late reply. The reason is, that I found your message in
> the MathGroup archive, but never recieved it by mail. In addition it
> puzzles me, why you were not able to find my post, since it is
> present in the archive ...
>
> Regarding your comment, I repeat, how I would use the symbol
> $Assumptions to define the function f to work for t without setting
> the upvalue to it:
>
> f[t_] := If[MemberQ[{$Assumptions}, t > 0], t, -t]
>
> Assuming[t > 0, f[t]]
>
> which will return t, if t is assumed to be positive and -t otherwise.
>
> The draw back is, that one also has to give a return value if t is not
> greater than 0.
>
> Best regards,
>
> Christoph
>
....

Hmmm, wouldn't g be the better choice of the two functions below?

In[1]:= f[t_] := If[MemberQ[{$Assumptions}, t > 0], t, -t]
In[2]:= Assuming[t^3 > 0, f[t]]
Out[2]= -t
In[3]:= g[t_] := If[FullSimplify[t > 0, $Assumptions], t, -t]
In[4]:= Assuming[t^3 > 0, g[t]]
Out[4]= t


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