#### Erland

Science Advisor

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But ##k## depends upon ##T## and there might be another ##k## for ##T^{-1}##.Since [itex]\mathbf{T^{-1}}[/itex] share the same property we have [itex]l(\mathbf{v})=l(\mathbf{T^{-1}}(\mathbf{T}\mathbf{(\mathbf{v}})))=kl(\mathbf{T}(\mathbf{v})=k^2l(\mathbf{v})[/itex] whence [itex]k^2=1[/itex]