Acknowledgements

We are grateful to Grzegorz Banaszak and Bartosz Naskręcki for discussions and suggestions. We wish to thank an anonymous referee for many improvements; in particular, for suggesting the concise formulation and proof of Lemma 5.

1

U. Storch, Bemerkung zu einem Satz von M. Kneser, Arch. Math. (Basel) 23 (1972), 403-404

2

D. Eisenbud, E. Evans, Every algebraic set in \(n\)-space is the intersection of \(n\) hypersurfaces, Invent. Math. 19 (1973), 107-112