Linear Fractional Transformation
A transformation of the form
w=f(z)=(az+b)/(cz+d),
(1)
where a, b, c, d in C and
ad-bc!=0,
(2)
is a conformal mapping called a linear fractional transformation. The transformation can be extended to the entire extended complex plane C^*=C union {infty} by defining
f(-d/c) = infty
(3)
f(infty) = a/c
(4)
(Apostol 1997, p. 26). The linear fractional transformation is linear in both w and z, and analytic everywhere except for a simple pole at z=-d/c.
Kleinian groups are the most general case of discrete groups of linear fractional transformations in the complex plane z->(az+b)/(cz+d).
Every linear fractional transformation except f(z)=z has one or two fixed points. The linear fractional transformation sends circles and lines to circles or lines. Linear fractional transformations preserve symmetry. The cross ratio is invariant under a linear fractional transformation. A linear fractional transformation is a composition of translations, rotations, magnifications, and inversions.
To determine a particular linear fractional transformation, specify the map of three points which preserve orientation. A particular linear fractional transformation is then uniquely determined. To determine a general linear fractional transformation, pick two symmetric points alpha and alpha_S. Define beta=f(alpha), restricting beta as required. Compute beta_S. f(alpha_S) then equals beta_S since the linear fractional transformation preserves symmetry (the symmetry principle). Plug in alpha and alpha_S into the general linear fractional transformation and set equal to beta and beta_S. Without loss of generality, let c=1 and solve for a and b in terms of beta. Plug back into the general expression to obtain a linear fractional transformation.