Schwarzian derivative

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In mathematics, the Schwarzian derivative is a certain operator that is invariant under all linear fractional transformations. Thus, it occurs in the theory of the complex projective line, and in particular, in the theory of modular forms and hypergeometric series.

1 Definition
2 Properties
3 Differential equation
4 Schwarzian derivatives as cocycles
5 Inversion formula
6 References

[edit] Definition

The Schwarzian derivative of a function of one complex variable $f$ is defined by

$(Sf)(z) = \left({f''(z) \over f'(z)}\right)' - {1\over 2}\left({f''(z)\over f'(z)}\right)^2$

$={f'''(z) \over f'(z)}-{3\over 2}\left({f''(z)\over f'(z)}\right)^2$

The alternate notation

$\{f,z\} = (Sf)(z)\,$

is frequently used.

[edit] Properties

The Schwarzian derivative of a linear fractional transformation

$g(z) = \frac{az + b}{cz + d}$

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

$(S(f\circ g))(z)=g'(z)^2(Sf)(g(z)).$

More generally for any sufficiently differentiable f and g

$S(f\circ g) = \left( S(f)\circ g\right ) \cdot(g')^2+S(g)$

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

$(Sf)(y)=6\lim_{x\rightarrow y} \left({f'(x)f'(y)\over(f(x)-f(y))^2}-{1\over(x-y)^2}\right).$

[edit] Differential equation

The Schwarzian derivative has a curious interplay with second-order linear ordinary differential equations. Let $f 1 (z)$ and $f 2 (z)$ be two linearly independent holomorphic solutions of

$\frac{d^2f}{dz^2}+ Q(z) f(z)=0$

Then the ratio $g (z) = f 1 (z) / f 2 (z)$ satisfies

$(Sg)(z) = 2Q(z)\,$

over the domain on which $f 1 (z)$ and $f 2 (z)$ are defined, and $f_2(z) \ne 0.$ The converse is also true: if such a g exists, and it is holomorphic on a simply connected domain, then two solutions $f 1$ and $f 2$ can be found, and furthermore, these are unique up to a common scale factor.

When a linear second-order ordinary differential equation can be brought into the above form, the resulting Q is sometimes called the Q-value of the equation.

Note that the Gaussian hypergeometric differential equation can be brought into the above form, and thus pairs of solutions to the hypergeometric equation are related in this way.

[edit] Schwarzian derivatives as cocycles

For a one-dimensional manifold M, let $F λ (M)$ be the space of tensor densities of degree $λ$ on M. The group of diffeomorphisms of M, Diff(M), acts on $F λ (M)$ via pushforwards. If f is an element of Diff(M) then consider the mapping

$f \rightarrow S(f^{-1})\,$ .

In the language of group cohomology the chain-like rule above says that this mapping is a 1-cocycle on $Diff(\mathbb{RP}^1)$ with coefficients in $F_2(\mathbb{RP}^1)$ . In fact

$H^1(Diff(\mathbb{RP}^1);F_2) = \mathbb{R}$

and the 1-cocycle generating the cohomology is f → S(f⁻¹).

There is an infinitesimal version of this result giving a 1-cocycle for the Lie algebra Vect( $\mathbb{RP}^1$ ) of vector fields. This in turn gives the unique non-trivial central extension of $V e c t (S 1)$ , the Virasoro algebra.