Date: May 12, 2009 12:37 PM
Author: Bill Dubuque
Subject: Re: -1 x -1 ?

hagman <google@von-eitzen.de> wrote:> On 12 Mai, 06:59, Kayama <yano...@earth.ocn.ne.jp> wrote:>>>> If we can use the expression -1=e^{i\pi}, we can show>> -1*-1=e^{i\pi}*e^{i\pi}=1 readily:>>>> if we rotate once 1 (the vector 01) by \pi rad around the origin anticlockwise on the complex plane, we obtain>> -1. Further likewise if we rotate -1 by \pi rad once more, we can obtain -1*-1=1.>>>> At least I understand -1*-1=1 in this way.>>>> In that way we can obtain i (imaginary unit), if we rotate 1 by \pi/2 around the origin anticlockwise.>>>> If we cannnot allow to use the expression -1=e^[i\pi}, I don't understand -1*-1=1 vividly.> > Well, (-1)*(-1) = 1 holds in all rings, not just in the field C.> And I doubt you can prove a lot about the exponential function> without making use of (-1)*(-1) = 1 somewhere ...More generally, the Law of Signs is simply the special linear case of the composition of odd functions in near-rings, see my 08 Feb 2008 posthttp://google.com/groups?selm=y8z1w7nf41w.fsf%40nestle.csail.mit.edu--Bill Dubuque