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MathGroup Archive 2002

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RE: 4.1 wont Solve LogNormal that 3.x could?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg35959] RE: [mg35923] 4.1 wont Solve LogNormal that 3.x could?
  • From: "DrBob" <majort at cox-internet.com>
  • Date: Fri, 9 Aug 2002 05:18:07 -0400 (EDT)
  • Reply-to: <drbob at bigfoot.com>
  • Sender: owner-wri-mathgroup at wolfram.com

No... my use of Simplify had nothing to do with it; I just wanted to see
simple answers.  My solution does things in a certain order and avoids
the step that Solve had trouble with.

The fact that Mathematica doesn't Simplify Log[Exp[x]] is just an
example of what might seem correct but isn't quite right; it's not
necessarily the specific problem in your situation.

Bobby

-----Original Message-----
From: Nicholas Georgakopoulos [mailto:georgakopoulos at mindspring.com] 
To: mathgroup at smc.vnet.net
Subject: [mg35959] Re: [mg35923] 4.1 wont Solve LogNormal that 3.x could?

Your code works even if all Simplify[ ] are removed.
Doesn't that negate the proposition that M'ca 4.1 refuses to solve due
to
proper refusal to simplify?


----- Original Message -----
From: "DrBob" <majort at cox-internet.com>
To: mathgroup at smc.vnet.net
Subject: [mg35959] RE: [mg35923] 4.1 wont Solve LogNormal that 3.x could?


> I don't know why 4.2 considered it unsolvable, but it could be because
> 4.2 knows more, not less.  4.2 is unwilling to simplify Log[E^x] to x,
> for instance, probably because of concerns about the many possible
> branches of Log.  In the error message a bad equation is listed that
> essentially equates two logarithms.  You or I would simplify by
> exponentiating each, but 4.2 knows that branch issues make that not
> quite right.
>
> Anyway, here's a solution in 4.2 that avoids the problem:
>
> Off[General::spell1, Solve::ifun]; s =. ; m =. ;
> sols = Solve[{Mean[LogNormalDistribution[m, s]] == mean}, m]
> v = Simplify[Variance[LogNormalDistribution[m, s]] /. First[sols]]
> Solve[{v == stdev^2}, s]//Simplify
>
> Bobby
>
> -----Original Message-----
> From: ng [mailto:georgakopoulos at mindspring.com]
To: mathgroup at smc.vnet.net
> Sent: Thursday, August 08, 2002 5:06 AM
> Subject: [mg35959] [mg35923] 4.1 wont Solve LogNormal that 3.x could?
>
> Why could 3.x sovle the following, while 4.1 considers it unsolvable?
> Thanks!
> Nicholas
>
> Off[General::spell1, Solve::ifun]; s =. ;m=.;
> sols = Solve[ {
> Mean[LogNormalDistribution[m, s]] == mean,
> Variance[LogNormalDistribution[m, s]] == stdev^2
> }, {m, s}] )
>
>
>
>
>







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