We show that, under some additional assumptions, all projection operators onto latticially closed subsets of the Orlicz-Musielak space generated by Φ are isotonic if and only if Φ is convex with respect to its second variable. A dual result of this type is also proven for antiprojections. This gives the positive answer to the problem presented in Opuscula Mathematica in 2012.
The properties of four elements (LPFE) and (UPFE), introduced by Isac and Persson, have been recently examined in Hilbert spaces, Lp-spaces and modular spaces. In this paper we prove a new theorem showing that a modular of form ρφ(∫)= ∫ Ω φ (t,/∫(t)/)dμ(t) satisfies both (LPFE) and (UPFE) if and only if φ is convex with respect to its second variable. A connection of this result with the study of projections and antiprojections onto latticially closed subsets of the modular space Lφ is also discussed.
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