Let U⊂RnU \subset \mathbb R^nU⊂Rn be open. A function f:U→Rmf: U\to \mathbb R^mf:U→Rm is of class CkC^kCk if its derivatives Df,D2f,…,DkfDf, D^2f,\dots, D^kfDf,D2f,…,Dkf all exist and are continuous on UUU.
Wikidata ID: Q2138760