On the Bertrandias-Payan module in a p-extension -- capitulation kernel (1510.05382v1)

Abstract: For a number field K and a prime number p we denote by BP_K the compositum of the cyclic p-extensions of K embeddable in a cyclic p-extension of arbitrary large degree. Then BP_K is p-ramified (= unramified outside p) and is a finite extension of the compositum K~ of the Z_p-extensions of K.We study the transfer map j_(L/K) (as a capitulation map of ideal classes) for the Bertrandias-Payan module bp_K:=Gal(BP_K/K~) in a p-extension L/K (p\textgreater{}2, assuming the Leopoldt conjecture). In the cyclic case of degree p, j_(L/K) is injective except if L/K is kummerian, p-ramified, non globally cyclotomic but locally cyclotomic at p (Theorem 3.1). We then intend to characterize the condition #bp_K divides #bp_L^G (fixed points). So we study bp_L^G when j_(L/K) is not injective and show that it depends on the Galois group (over K~) of the maximal Abelian p-ramified pro-p-extension of K.We give complete proofs in an elementary way using ideal approach of global class field theory.