The Fundamental Theorem of Similarity Geometry
If L = (A,B) and L' = (A',B') are lists of distinct points of E, then
there is a unique element of S+(2) which maps L to L'.
Let A, B, A', B' have complex coordinates α, β, α', β' respectively.
An element s of S+(2) has the form s(z) = γz+δ, with γ ≠ 0.
This maps L to L' if and only if γα + β = α', and γβ + δ = β'.
As A, B are distinct α ≠ β, and as A', B' are distinct, α' ≠ β'.
As (α - β) ≠ 0, the equations have the unique solution
γ = (α' - β')/(α - β) and δ = (αβ' - α'β)/(α-β).
As α' ≠ β', γ ≠ 0, so s(z) = γz + δ is in S+(2).
Thus we have a unique element of S+(2) mapping L to L'.
This is a purely algebraic proof, and gives no idea how the map can be constructed.
We begin with a dilation d about A mapping B to C, where |AC| = |A'B'|.
Unfortunately, the uniqueness of s is not at all clear!
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