Gary
GaryVasco
Posts: 3,352
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Post by Gary on Oct 21, 2016 6:30:43 GMT
Vasco, I have plotted and pointed out some similarities between the three types of direct motion introduced in this chapter and the Mobius transformations introduced in chapter 3. The point of this was just to see if I could reproduce the plots in this section and study the framework at the same time. I haven't figured out yet how to use the formulas given in the quote on p. 309. Suppose you want an h-circle with radius $\rho$ centred at a particular spot. Do we have enough information to plot it? h.and.euclid.circles.rv.pdf (711.56 KB) Gary
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Post by Admin on Oct 24, 2016 14:40:46 GMT
Gary
Here is some feedback on your document:
In your first paragraph I think I understand what you mean by the $z$-plane and $w$-plane, but referring to the LHS and RHS as the $z$-plane and $w$-plane could potentially be confusing as in figure 29 on page 163 $z$ and $w$ are both on the LHS and $\widetilde{z}$ and $\widetilde{w}$ are both on the RHS. In figure 1a you have written $w$ and $\widetilde{M}(w)$ and in 1b you have written $z=F^{-1}(w)$, but in the book Needham has $w=M(z)$ in LHS of figure 29 and $\widetilde{w}=\widetilde{M}(\widetilde{z})$ in RHS of figure 29. The word hyperbolic on page 153 is not related to the ideas of the hyperbolic plane, as I understand it - see footnote on page 308.
Discussion of h-rotation on pages 2-3 Since the fixed points $\xi_+$ and $\xi_-$ can be any two points then we can choose $\xi_+=\alpha i$ and $\xi_-=-\alpha i$ where $\alpha$ is real and positive and then the picture will be like your figure 2 on page 2. As I understand things the Möbius transformation $F$ is not the one we are interested in here, but the one called $M$ which transforms $z$ to $w$ on the LHS of figures 29, 34, 35 and 36 of chapter 3. I see the RHS pictures as being there to help us understand what is happening on the LHS. On page 3 you talk about the figures 1a and 1b having fixed points, but it's the Möbius transformation $M$ which has fixed points at $\xi_+$ and $\xi_-$, and $\widetilde{M}$ which has fixed points at 0 and $\infty$. For me the fact that $M$ is elliptic is not determined by $F$, but is a characteristic of $M$ itself.
If you look at (46) on page 173, you will see that the Möbius transformation $M$ is the composition of reflections in any two circles which pass through the fixed points. It then follows from (38) and (37) on page 303 that the h-rotation about $a$ is a perfect rotation (to a Poincarite), as described on pages 308-9.
I think your paragraph entitled Limit-translation should be H-translation from looking at figure 3 on page 3 and comparing it with figure 30 on page 311.
Vasco
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Gary
GaryVasco
Posts: 3,352
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Post by Gary on Oct 24, 2016 15:09:11 GMT
Gary Here is some feedback on your document: In your first paragraph I think I understand what you mean by the $z$-plane and $w$-plane, but referring to the LHS and RHS as the $z$-plane and $w$-plane could potentially be confusing as in figure 29 on page 163 $z$ and $w$ are both on the LHS and $\widetilde{z}$ and $\widetilde{w}$ are both on the RHS. ... Vasco, Quite right on all points. I have revised. Gary
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