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Re: Not all points plot on my graph...

  • To: mathgroup at smc.vnet.net
  • Subject: [mg105244] Re: Not all points plot on my graph...
  • From: dh <dh at metrohm.com>
  • Date: Wed, 25 Nov 2009 02:34:21 -0500 (EST)
  • References: <hedte4$lsn$1@smc.vnet.net>


davef wrote:

> Why is it when I write this...

> 

> Clear[f, x]

> f[x_] = (x + 2)^(2/3)

> Plot[f[x], {x, -3, 0}, PlotRange -> {{-3, 0}, {-4, 4}}]

> f[-3]

> 

> .. I get no values show in the graph for x<-2 even though f[-3]

> evaluates to (-1)^(2/3).

> 

> (-1)^(2/3) evaluates as f[-3]=1 so why wouldn't the point (-3,1) plot

> on my graph?

> 

> 

Hi Dave,

your function is multivalued. The principal value is complex for 

arguments below -2. Mathematica returns the principal value for numeric 

functions. Read on the internet how the principal value is defined.

Where did you see that (-1)^(2/3) evaluates to 1?? This is not the 

principal value.

Daniel




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