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MathGroup Archive 2006

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Re: Infinity vs DirectedInfinity[1]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg70538] Re: [mg70489] Infinity vs DirectedInfinity[1]
  • From: Carl Woll <carlw at wolfram.com>
  • Date: Thu, 19 Oct 2006 03:21:53 -0400 (EDT)
  • References: <200610180816.EAA07979@smc.vnet.net>

Chris Chiasson wrote:

>Infinity is a symbol, as far as I know. However, in FullForm it is
>shown as the equivalent DirectedInfinity[1].
>
>Is there some kind of MakeBoxes rule for FullForm that makes Infinity
>show up that way?
>
>  
>
Actually, when evaluated, Infinity is converted to DirectedInfinity[1]. 
This is like when I is converted to Complex[0,1]. Compare:

FullForm[Hold[Infinity]]
Hold[Infinity]

and

FullForm[Infinity]
DirectedInfinity[1]

On the other hand, DirectedInfinity[d] has formatting rules for 
InputForm, StandardForm and TraditionalForm that cause it to be 
displayed as Sign[d] Infinity when d is +/-1 or +/-I.

Carl Woll
Wolfram Research


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