Extrapolation et synthèse

Let G a locally compact group, H a closed subgroup and 1 < p < ∞. It's well-known that the restriction of the functions from G to H is a surjective linear contraction from Ap(G) onto Ap(H). We prove, when H is amenable, that every element in Ap(H) with compact support can be extended to an element in Ap(G) of which we can check norm and support. This result is already known in the case of normal subgroups and also for compact subgroups. We obtain the existence of a quasi-coretract in the BAN category, as a substitute of a morphism ΓH such that ResH ◦ ΓH = idAp(H). Indeed, for an amenable subsgroup, the morphism ΓH, a priori, doesn't exist. So, we construct a net of morphismes in BAN from Ap(H) into Ap(G), that converge to idAp(H) for the strong operator's topology on Ap(H) (that's for us the notion of a quasi-coretract in BAN). Furthermore, if H is metrizable and σ-compact we obtain, more precisely, a sequence. Moreover, our approach allows us to extend to the non-abelian case some works of H. Reiter and C. Herz concerning the spectral synthesis of bounded uniformly continuous functions. My results are new even for the Fourier algebra.