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arXiv 1204.2133math.NT
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局部域弱分歧扩张的显式积分Galois模结构

Explicit integral Galois module structure of weakly ramified extensions of local fields

Henri Johnston

摘要:

设L/K是完备局部域的有限Galois扩张,其剩余域有限,G=Gal(L/K).设G_1和G_2是第一和第二分歧群。当G_1是平凡的时,L/K是弱分歧的;当G_2是平凡的时,L/K是弱分歧的。设O_L是L的赋值环,P_L是它的极大理想.证明了若L/K是弱分歧的,且n同余于1 mod |G_1|,则P_L^n在群环O_K[G]上是自由的,并构造了一个显式生成元.在L/K是广分歧的假设下,证明了O_K[G]上P_L的每一个自由生成元也是群代数K[G]中O_L在其相关阶上的自由生成元.一路上,我们证明了一个“分裂引理”的局部域,这可能是独立的利益。

英文摘要:

Let L/K be a finite Galois extension of complete local fields with finite residue fields and let G=Gal(L/K). Let G_1 and G_2 be the first and second ramification groups. Thus L/K is tamely ramified when G_1 is trivial and we say that L/K is weakly ramified when G_2 is trivial. Let O_L be the valuation ring of L and let P_L be its maximal ideal. We show that if L/K is weakly ramified and n is congruent to 1 mod |G_1| then P_L^n is free over the group ring O_K[G], and we construct an explicit generating element. Under the additional assumption that L/K is wildly ramified, we then show that every free generator of P_L over O_K[G] is also a free generator of O_L over its associated order in the group algebra K[G]. Along the way, we prove a `splitting lemma' for local fields, which may be of independent interest.

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