Explicit formulas for \(C^{1,1}\) Glaeser-Whitney extensions of 1-fields in Hilbert spaces

We give a simple alternative proof for the \(C^{1,1}\)--convex extension problem which has been introduced and studied by D. Azagra and C. Mudarra [2]. As an application, we obtain an easy constructive proof for the Glaeser-Whitney problem of \(C^{1,1}\) extensions on a Hilbert space. In both cases we provide explicit formulae for the extensions. For the Gleaser-Whitney problem the obtained extension is almost minimal, that is, minimal up to a factor \(\frac{1+\sqrt{3}}{2}\) in the sense of Le Gruyer [15].