Regular value

2020 Mathematics Subject Classification: Primary: 57R [MSN][ZBL]

of a differentiable function $f: \mathbb R^m \to \mathbb R^n$ with $n\leq m$. A value $c\in \mathbb R^n$ such that the differential of $f$ is surjective at every preimage of $c$. The concept can be readily extended to the case when the domain and the target are smooth manifolds and to various infinite-dimensional settings.