Schwarzian derivative

There are many ways the basic derivative can be generalized: partial derivatives, directional derivatives, covariant derivatives, etc. These all reduce to the basic derivative under the right circumstances.

The Schwarzian derivative is not like that. It’s not a generalization of the familiar derivative but rather a differential operator analogous to a derivative. The Schwarzian derivative of a function f is defined [1] as

S(f) = \left(\frac{f''}{f'}\right)' - \frac{1}{2} \left(\frac{f''}{f'}\right)^2

To understand the motivation behind such an arbitrary-looking definition, we need to first look at functions of the form

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

called Möbius transformations, or more descriptively, fractional linear transformations [2]. These transformations are very important in complex analysis and have a lot of interesting properties. For example, the image of a circle in the complex plane under a Möbius transformation is another circle [3].

Möbius transformations are to the Schwarzian derivative roughly what constants are to the ordinary derivative. A function is a Möbius transformation if and only if its Schwarzian derivative is zero.

Since the Schwarzian derivative is defined in terms of ordinary derivatives, adding a constant to a function doesn’t change its Schwarzian derivative. Furthermore, the Schwarzian derivative is defined in terms of the ratio of ordinary derivatives, so multiplying a function by a constant doesn’t change its Schwarzian derivative either.

Even more generally, applying a Möbius transformation to a function doesn’t change its Schwarzian derivative. That is, for a fractional linear transformation like g(z) above

S(g \circ f) = S(f)

for any function f. So you can pull Möbius transformations out of a Schwarzian derivative sorta like the way you can pull constants out of an ordinary derivative. The difference though is that instead of the Möbius transformation moving to the outside, it simply disappears.

You can think of the Schwarzian derivative as measuring how well a function can be approximated by a Möbius transformation. Schwarzian derivatives come up frequently in applications of complex analysis, such as conformal mapping.

More kinds of derivatives

[1] The Schwarzian derivative of a constant function is defined to be zero.

[2] Möbius transformations require adbc to not equal zero.

[3] For this to always be true, you have to include a line as a special case of a circle, a circle of infinite radius if you like. If you don’t like that definition, then you can rephrase the statement above as saying Möbius transformations map circles and lines to circles and lines.

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