Re: Problem with eval. of neg. cube root of neg. #

*To*: mathgroup at smc.vnet.net*Subject*: [mg49935] Re: [mg49894] Problem with eval. of neg. cube root of neg. #*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Fri, 6 Aug 2004 03:10:02 -0400 (EDT)*Reply-to*: hanlonr at cox.net*Sender*: owner-wri-mathgroup at wolfram.com

Plot[Sign[x]*Abs[x]^(1/3)*(x + 4), {x, -10, 10}, PlotRange -> All]; Bob Hanlon > > From: pootleguard-mathgroup at yahoo.com (Josh) To: mathgroup at smc.vnet.net > Date: 2004/08/05 Thu AM 09:21:47 EDT > To: mathgroup at smc.vnet.net > Subject: [mg49935] [mg49894] Problem with eval. of neg. cube root of neg. # > > I am having trouble plotting the following function: > > Plot[x^(1/3)*(x + 4), {x, -10, 10}] > > Mathematica won't plot this function for negative x, although it is > obviously defined for negative x. It seems to be evaluating the > negative part of this function to imaginary numbers for some odd > reason.If I do: > > f[x_] := (x^(1/3))*(x + 4) > > and then: > > f[-5] // N > > I get: > > -0.854988 - 1.48088 \[ImaginaryI] > > when the correct answer is the negative cube root of negative 5, which > is approximately - (-1.70998) = 1.70998 > > I can send a copy of the notebook that shows where this is happening > to anyone who requests it... > > Can anyone explain what is going on here? Is this a bug or am I > missing something? > > Thanks in advance for any help ... > >