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I know that if complex numbers $z$ and z' correspond to opposite points on the Riemann sphere, then it must be the case that z\bar{z}'=-1.

Is the converse true, that z\bar{z}'=-1 implies that the corresponding points on the Riemann sphere are opposite points?

I associate $(x_1,x_2,x_3)$ with $z$ and (x'_1,x'_2,x'_3) with z'. The usual correspondence gives z=\frac{x_1+ix_2}{1-x_3},\;\;\;\;\; z'=\frac{x'_1+ix'_2}{1-x'_3}. Then I reach an equation z\bar{z}'=\frac{x_1+ix_2}{1-x_3}\cdot\frac{x'_1-ix'_2}{1-x'_3}=-1 which implies x_1x'_1+x_2x'_2+x_3x'_3+(x'_1x_2-x_1x'_2)i=-1+x_3+x'_3. Can one conclude that (x_1,x_2,x_3)=-(x'_1,x'_2,x'_3) from this relation? Thank you.

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There's only one possible complex $w$ for which $zw$ can equal $-1$, given by $w = -{1 \over z}$. So since z\bar{z'} = -1 for $z$ and z' corresponding to antipodal points on the Riemann sphere, if $z\bar{w} = -1$ as well you'd have to have \bar{w} = \bar{z'} or w = z'.

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    Oops, that's simple. Thanks.2012-01-27