 Schwarzian derivative

For a thrice differentiable function with $u^{\prime }\ne 0$, the Schwarzian derivative isIt vanishes exactly for fractional linear functions $u(x)=(ax+b)/(cx+d)$ on intervals where the denominator does not vanish. In a two-dimensional conformal field theory, it is the inhomogeneous term in the finite conformal transformation of a stress-energy tensor with nonzero central charge.

The Schwarzian derivative is a concept from complex analysis and differential geometry that arises in the study of conformal mappings and holds significant importance in the theory of univalent (or schlicht) functions.