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Re: Truth in inequalities

  • To: mathgroup at
  • Subject: [mg29375] Re: Truth in inequalities
  • From: "Orestis Vantzos" <atelesforos at>
  • Date: Fri, 15 Jun 2001 02:23:44 -0400 (EDT)
  • Organization: National Technical University of Athens, Greece
  • References: <9g9mra$fpt$>
  • Sender: owner-wri-mathgroup at

I am afraid I have to agree with you...x might as well be Infinity in a
future evaluation of the x-Infinity expression, and so can not be readily
A possible correction could be to use the following rule:
Infinity + n_?NumericQ -> Infinity
Orestis Vantzos
PS. The rule does not work for Infinity+Infinity since
It does simplify to 2*Infinity though, which can be handled by a different

"Jack Goldberg" <jackgold at> wrote in message
news:9g9mra$fpt$1 at
> Hi group,
> Can someone explain the logic of the following:
> x < Infinity   returns x < Infinity
> while
> x - Infinity < 0  returns  True
> I should mention that I am aware of the fact that x - Infinity simplifies
> automatically to -Infinity which is then compared to 0 and found wanting.
> The issue I'm raising is why should a CAS that has  x-Infinity < 0 return
> True not also return True for the  x < Infinity?  One awkwardness of
> having this difference of behavior can be seen in the example,
> MyFunction[x_,y_]/;(x<y) :=  blah
> and
> MyFunction[x_,y_]/;(x-y<0)  := blah
> do not do the same thing when, say, y=Infinity.
> Just curious :-)
> Jack

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