Zu einer Abbildung

${\displaystyle \varphi \colon L\longrightarrow M}$

${\displaystyle {}L}$ und ${\displaystyle {}M}$ heißt zu ${\displaystyle {}P\in L}$ die Menge

${\displaystyle F_{P}={\left\{x\in L\mid \varphi (x)=\varphi (P)\right\}}}$

${\displaystyle {}\varphi }$

${\displaystyle {}P}$