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MathGroup Archive 2001

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Re: Limit[ Sign[],x->Infinity ]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27286] Re: [mg27289] Limit[ Sign[],x->Infinity ]
  • From: BobHanlon at aol.com
  • Date: Sat, 17 Feb 2001 03:31:04 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Why would you take the limit rather than just indicate that x is positive?

Simplify[Sign[x]*Sign[z], x > 0]

Sign[z]

Simplify[Sign[x]+Sign[z], x > 0]

Sign[z] + 1


Bob Hanlon

In a message dated 2001/2/16 4:27:39 AM, jackgold at math.lsa.umich.edu writes:

>I give you two examples using ver. 4.0 on a Mac and a unix system.
>
>   Limit[ Sign[x]*Sign[z],x->Infinity ]  
>
>yields Sign[z]  which is good.
>
>   Limit[ Sign[x]+Sign[z],x->Infinity] 
>
>yields an expression too ugly to present to a wide audience, which is bad.
>
>Its not a bug since 
>
>   Limit[ Sign[x]+Sign[z],x->Infinity]//FullSimplify
>
>yields 1+Sign[z] which is good.
>
>I propose the name "buglet" to describe such "features" :-).  By the way,
>I tried some obvious variations on combinations of Sign[z] and Sign[x]
>and
>found it pretty hard to predict the output. 
>


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