= Schwarzian derivative
{c}
{title2=$S[u]=\{u,x\}$}
{wiki}
For a thrice differentiable function with $u'\ne0$, the Schwarzian derivative is
$$
S[u]=\frac{u'''}{u'}-\frac32\left(\frac{u''}{u'}\right)^2.
$$
It 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.
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