Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2006
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2006

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Limit and Root Objects

  • To: mathgroup at smc.vnet.net
  • Subject: [mg72285] Re: Limit and Root Objects
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Mon, 18 Dec 2006 06:56:00 -0500 (EST)
  • References: <B6993FE7-D191-4A9D-9741-C4CA161BA14A@akikoz.net>

On 18 Dec 2006, at 17:35, Andrzej Kozlowski wrote:

> It is easy to check that the function
>
>
> f[b_] := Root[#1^3 + b*#1 - 1 & , 1]
>
> is discontinuous at b, where
>
>
> Reduce[Resultant[x^3 + b*x - 1, D[x^3 + b*x - 1, x], x] == 0, b,  
> Reals]
>
> b == -(3/2^(2/3))
>
> indeed this was not so long ago discussed in connection with  a  
> little argument about "usefulness' of Root objects. In view of  
> this, isn't the following a bug?
>
>
> u = Limit[f[b], b -> -(3/2^(2/3)), Direction -> 1]
>
>
> Root[2*#1^3 + 1 & , 1]
>
>
> v = Limit[Root[#1^3 + b*#1 - 1 & , 1], b -> -(3/2^(2/3)), Direction  
> -> -1]
>
>
> Root[2*#1^3 + 1 & , 1]
>
>
> u == v
>
> True
>
> It looks like Limit is making life too easy for itself by assuming  
> continuity.
>
> Using NLimit shows that things are not as simple:
>
>
> w = NLimit[f[b], b -> -(3/2^(2/3)), Direction -> -1]
>
> 1.5874010343874532
>
>
> z = NLimit[Root[#1^3 + b*#1 - 1 & , 1], b -> -(3/2^(2/3)),  
> Direction -> 1]
>
>
> -0.7937180869283765
>
> Andrzej Kozlowski


I forgot to add that the correct value of

v = Limit[Root[#1^3 + b*#1 - 1 & , 1], b -> -(3/2^(2/3)), Direction - 
 > -1]

is  Root[#1^3 - 4 & , 1]  (and not Root[2*#1^3 + 1 & , 1]).


Andrzej Kozlowski 


  • Prev by Date: Re: Problems with Set, SetDelayed and replacement rules...
  • Next by Date: Re: image border
  • Previous by thread: Limit and Root Objects
  • Next by thread: Re: Limit and Root Objects