RE: Re: Mathematica 4.2 & Strange Plot results

*To*: mathgroup at smc.vnet.net*Subject*: [mg36386] RE: [mg36328] Re: Mathematica 4.2 & Strange Plot results*From*: "DrBob" <drbob at bigfoot.com>*Date*: Wed, 4 Sep 2002 02:56:43 -0400 (EDT)*Reply-to*: <drbob at bigfoot.com>*Sender*: owner-wri-mathgroup at wolfram.com

Plot[Evaluate[Sin[x]/Sin[x]], {x, -2.5, -1.5}] Gives the "expected" graph. Bobby Treat -----Original Message----- From: Ian McInnes [mailto:ian at whisper-wood.demon.co.uk] To: mathgroup at smc.vnet.net Subject: [mg36386] [mg36328] Re: Mathematica 4.2 & Strange Plot results This results from: a) the fact that Mathematica implements division as multiplication and reciprocation b) the use of machine precision for plotting the graph c) the default scaling of the y axis revealing the resultant inaccuracies Although Sin[x]/Sin[x] gives 1 when evaluated symbolically (x undefined), substituting a floating-point value for x results in composite division being performed numerically, with consequent inaccuracy. This can be verified by the following: In[1]:= FullForm[HoldForm[Sin[x]/Sin[x]] Out[1]//FullForm= HoldForm[Times[Sin[x], Power[Sin[x], -1]]] By default, Mathematica scales the y axis such that the small discrepancies are visible. The PlotRange option may be given to explicitly specify the range for the y axis (for example PlotRange->{0, 2}). Regards, Ian McInnes. "Ken Thomson" <someone at somewhere.com> wrote in message news:akn016$ljr$1 at smc.vnet.net... > I would expect that > > Plot[Sin[x]/Sin[x], {x, -2.5, -1.5}] > > would produce a horizontal line at y=1. > > However, on my Windows XP computer it produces a graph where the y value is > less than 1 at several points. Most notibly between x=-1.6 and x=-1.8 > > Is this just an isolated case? Or does it happen to others? If so - why? > > Regards, > Ken. > > >