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Re: assuming certain properties about variables

  • To: mathgroup at smc.vnet.net
  • Subject: [mg111437] Re: assuming certain properties about variables
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Sun, 1 Aug 2010 04:57:43 -0400 (EDT)

But once you put the restriction on the argument of f, you've told 
Mathematica not to carry out the evaluation of f unless the input 
supplied is actually positive.

If now t is just a symbol, then it is not positive (and not negative, 
either) -- it's just a symbol. So why would you expect to be able to 
tell Mathematica that it's positive?  In general, Mathematica variables 
don't really have types.

If it's just the particular symbol t that you want to supply to f, then 
you could do this (I'm changing your function definition for clarity) -- 
not that you probably want SetDelayed (:=) instead of Set (=):

   f[x_?Positive] := x^2
   f[t] = t^2;

   f[3]
9
   f[t]
t^2
   t = -5;
   f[t]
f[-5]

   f


On 7/31/2010 2:40 AM, Benjamin Hell wrote:
>
> let's say I have defined the following function:
> f[x_?Positive] = x
> Now I want to evaluate f with a variable t:
> f[t]
> As mathematica knows nothing about t, the output is f[t] instead of t.
>
> How can I tell mathematica, that t should be a positive number so that
> Positive[t] evaluates true and then f[t] evaluates to t?
> Of course this is just an example, which should present what I would
> like to know.

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305


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