Schwarzian Derivative

In mathematics, the Schwarzian derivative of a function of one complex variable f is defined by
(Sf)(z) = {f'(z) \over f'(z)}-{3\over 2}\left({f(z)\over f'(z)}\right)^2.
The Schwarzian derivative of a linear fractional transformation
g(z)=(az+b)/(cz+d)
is zero. If we follow a function f by a fractional linear transformation g then the composition g\circ f has the same Schwarzian derivative as f. On the other hand the Schwarzian derivative of f\circ g, where g is again fractional linear, is given by the remarkable chain-like rule
(S(f\circ g))(z)=g'(z)^2(Sf)(g(z)).
Just as the ordinary derivative tells us how a function can be approximated by a linear function, the Schwarzian derivative tells us how a function can be approximated by a fractional linear function. The Schwarzian derivative can also be defined as the following limit
(Sf)(y)=6\lim_{x\rightarrow y} \left({f'(x)f'(y)\over(f(x)-f(y))^2}-{1\over(x-y)^2}\right).

References

  • V. Ovsienko, S. Tabachnikov : Projective Differential Geometry Old and New, Cambridge University Press, 2005. ISBN 0521831865 .

 

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