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Re: Re: The graph of (x + 2)^(1/5) + 4 not plotted correctly

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  • Subject: [mg103938] Re: [mg103926] Re: The graph of (x + 2)^(1/5) + 4 not plotted correctly
  • From: Murray Eisenberg <murray at>
  • Date: Tue, 13 Oct 2009 23:17:45 -0400 (EDT)
  • Organization: Mathematics & Statistics, Univ. of Mass./Amherst
  • References: <> <>
  • Reply-to: murray at

Could you please just tell us what you think is wrong with the plot that 
Mathematica provides?

I ask because when I point my browser to the link you provide...

(1) I get a large number of annoying (and potentially dangerous) 
3rd-party ad-generating items on the URL page that clamor to be allowed 
to do their thing.

(2) The plot finally displayed is of something entirely different -- of 
exp(-x^2) and its first two derivatives.

As to the function (x + 2)^(1/5) + 4 that you asked about: Did you just 
try something like the following?

   Plot[(x + 2)^(1/5) + 4, {x,-7,5}]

If so, then you were probably surprised that the graph ended on the left 
at x == -2.  And that is precisely what Mathematica should be expected 
to do. After all, Mathematica "wants" numbers to be complex when they 
can be.  And z^(1/5) denotes the PRINCIPAL 5th-root of z, so that when z 
is negative, you do NOT get a negative real number, but rather the 
non-real, complex principal 5th-root.

If you want the high school/calculus variety 5th-root function, then you 
need to finesse this. For example:

   f[x_] := Sign[x + 2] Abs[x + 2]^(1/5) + 4
   Plot[ f[x], {x,-7,5}]

(You could use Piecewise instead.)

victor chg wrote:
> Here is the link to the image:
> Victor Chg

Murray Eisenberg                     murray at
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305

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