Re: Forcing Re[]'s to be Real

*To*: mathgroup at smc.vnet.net*Subject*: [mg18239] Re: Forcing Re[]'s to be Real*From*: "David Keith" <dkeith at sarif.com>*Date*: Thu, 24 Jun 1999 14:24:20 -0400*Organization*: Hevanet Communications*References*: <7kpbj1$4b8@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi Anthony, Two things I think: 1) Your definition assigned to k[w] instead of k[w_], 2) Re[] doesn't seem to want to approximate the exact number Sqrt[Complex], but Re[Complex]//N ans Re[ApproximateComplex] works fine. Since plot uses N[] it seems to work fine also. Dave See below: In[295]:= k[w_] := Sqrt[w^2 (1 + (2 / (1 + I w)))] In[305]:= Re[k[1]] Out[305]= \!\(Re[\ at \(2 - \[ImaginaryI]\)]\) In[306]:= Re[k[1.]] Out[306]= 1.45535 In[298]:= Re[k[1]] // N Out[298]= 1.45535 In[299]:= Im[k[1]] // N Out[299]= -0.343561 Plot[{Re[k[x]], Im[k[x]]}, {x, -5, 5}] Anthony Foglia wrote in message <7kpbj1$4b8 at smc.vnet.net>... > I seem to have found an interesting problem involving computing the >real part of complex numbers. (Interesting, in that it wasn't there a few >weeks ago when I ran the (as-far-as-i-can-remember) exact same code.) > >I have a complex function: > >k[w] := Sqrt[w^2 (1 + (2 / (1 + I w)))] > >I want to graph the real and imaginary parts, but Mathematica doesn't want >to express the Re[k[w]] as a real number. What do I mean? Well, if I >type: > >Re[Sqrt[1+I]] > >I get out > >Re[Sqrt[1+I]] > >Same if I do Re[ComplexExpand[Sqrt[1+I]]], or Re[(1+I)^(1/2)]. But if I >enter: > >Re[ComplexExpand[(1+I)^(1/2)] > >Mathematica is kind enough to respond with: > >2^(1/4) Cos[Pi/8] > >I'm certain that this is the root of my problem, but I'll be damned if I >know why Mathematica doesn't like it now, but did a few weeks ago. Any >help? > >--Anthony >