Schwarzian
Noun
Meaning
(singular only) A particular operator that, when applied to a function f yields a function
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{\displaystyle x\mapsto {\frac {f'''(x)}{f'(x)}}-{\frac {3}{2}}\left({\frac {f''(x)}{f'(x)}}\right)^{2}}
(countable) Any image of that operator.
Source: en.wiktionary.org