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MathGroup Archive 2000

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Re: entering an equation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg21606] Re: [mg21592] entering an equation
  • From: BobHanlon at aol.com
  • Date: Sun, 16 Jan 2000 22:43:52 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

For the function Log[2, Log[2, x]] to be real, Log[2, x] must be positive.

Log[2, x] > 0

Log[x]/Log[2] > 0

That is,

Log[x] > 0;

or

x > Exp[0]

x > 1

Needs["Algebra`InequalitySolve`"]

InequalitySolve[Log[2, x] > 0, x]

InequalitySolve::npi  A nonpolynomial equation or inequality encountered. The 
solution set may be incorrect.

x > 1

Table[{x, Log[2, Log[2, x]]}, {x, 0., 1.5, .25}]

{{0., Indeterminate}, {0.25, 1. + 4.532360141827193*I}, 
  {0.5, 0. + 4.532360141827193*I}, 
  {0.75, -1.2686864028144482 + 4.532360141827193*I}, 
  {1., Indeterminate}, {1.25, -1.6351896075841128}, 
  {1.5, -0.773583951828313}}

Plot[Log[2, Log[2, x]], {x, 1, 10}];

Bob Hanlon

In a message dated 1/16/2000 5:18:21 AM, mark at ratbyte.com writes:

>I am trying to plot a series of functions in Mathematica...one is giving
>me
>trouble. I need : ln2 (ln2 x)
>
>    I enter: Log[2, Log[2,x]]
>
>and I get:
>
>is not a machine-size real \
>number at \!\(x\) = \!\(4.1666666666666667`*^-7\)."
>
>Does anyone have any suggestions?
>


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