Ramification of local fields with imperfect residue fields II

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1 Ramification of local fields with imperfect residue fields II Ahmed Abbes and Takeshi Saito October 31, 2002 In the previous paper [1], a filtration by ramification groups and its logarithmic version are defined on the absolute Galois group of a complete discrete valuation field K without assuming that the residue field is perfect. In this paper, we study the graded pieces of these filtrations and show that they are abelian except possibly in the absolutely unramified and non-logarithmic case. Let G j K (j>0, Q) denote the decreasing filtration by ramification groups and Gj K,log (j> 0, Q) be its logarithmic variant. We put G j+ K = j >j Gj K show that the wild inertia subgroup P G K is equal to G 1+ K = G0+ K,log following. and Gj+ K,log = j >j Gj K,log. In [1], we. The main result is the Theorem 1 Let K be a complete discrete valuation field. 1. (see Theorem 2.14) Assume either K has equal characteristics p > 0 or K has mixed characteristic and p is not a prime element. Then, for a rational number j>1, the graded piece Gr j G K = G j K /Gj+ K is abelian and is a subgroup of the center of the pro-p-group G1+ K /Gj+ K. 2. (see Theorem 5.11) For a rational number j>0, the graded piece Gr j log G K = G j K,log /Gj+ K,log is abelian and is a subgroup of the center of the pro-p-group G 0+ K,log /Gj+ K,log. The idea of the proof of 1 is the following. Under some finiteness assumption, denoted by (F), we define a functor X j from the category of finite étale K-algebras with ramification bounded by j+ to the category of finite étale schemes over a certain tangent space Θ j with continuous semi-linear action of G K. For a finite Galois extension L of K with ramification bounded by j+, the image X j (L) has two mutually commuting actions of G = Gal(L/K) and G K. The arithmetic action of G K comes from the definition of the functor X j and the geometric action of G is defined by functoriality. Using these two commuting actions, we prove the assertion. In Section 1, for a rational number j>0 and a smooth embedding of a finite flat -algebra, we define its j-th tubular neighborhood as an affinoid variety. We also define its j-th twisted reduced normal cone. We recall the definition of the filtration by ramification groups in Section 2.1 using the notions introduced in Section 1. In the equal characteristic case, under the assumption (F), we define a functor X j mentioned above in Section 2.2 using j-th tubular neighborhoods. In the mixed characteristic case, we give a similar but subtler construction using the twisted normal cones, assuming further that the residue characteristic p is not a prime element of K in Section 2.3. Then, 1

2 we prove Theorem 2.14 in Section 2.4. We also define a canonical surjection π1 ab (Θ j ) Gr j G K under the assumption (F). After some preparations on generalities of log structures in Section 3, we study a logarithmic analogue in Sections 4 and 5. We define a canonical surjection π1 ab (Θ j log ) Grj log G K under the assumption (F) and prove the logarithmic part, Theorem 5.11, of the main result in Section 5.2. Among other results, we compare the construction with the logarithmic construction given in [1] in Lemma We also prove in Corollary 4.12 a logarithmic version of [1] Theorem 7.2 (see also Corollary 1.15). In Section 6, assuming the residue field is perfect, we show that the surjection π ab Gr j log G K induces an isomorphism π ab,gp 1 (Θ j ) Gr j log G K where π ab,gp 1 (Θ j log classifying the étale isogenies to Θ j log regarded as an algebraic group. 1 (Θ j log ) ) denotes the quotient When one of the authors (T.S.) started studing mathematics, Kazuya Kato, who was his adviser, suggested to read [13] and to study how to generalize it when the residue field is no longer assumed perfect. The authors are very happy to dedicate this paper to him for his 51st anniversary. Notation. Let K be a complete discrete valuation field, be its valuation ring and F be its residue field of characteristic p>0. Let K be a separable closure of K, O K be the integral closure of in K, F be the residue field of O K, and G K = Gal( K/K) be the Galois group of K over K. Let π be a uniformizer of and ord be the valuation of K normalized by ordπ = 1. We denote also by ord the unique extension of ord to K. 1 Tubular neighborhoods for finite flat algebras For a semi-local ring R, let m R denote the radical of R. We say that an -algebra R is formally of finite type over if R is complete, semi-local, Noetherian and the quotient R/m R is finite over F. We say that an -algebra R is topologically of finite type over if R is π-adically complete, Noetherian and the quotient R/πR is of finite type over F. For an -algebra R formally of finite type over, we put ˆΩ R/OK = lim nω (R/m n R )/. For an -algebra R topologically of finite type over, we put ˆΩ R/OK = lim nω (R/π n R)/. Here and in the following, Ω denotes the module of differential 1-forms. For a surjection R R of rings, its formal completion is defined to be the projective limit R = lim nr/(ker(r R )) n. In this section, A will denote a finite flat -algebra. 1.1 Embeddings of finite flat algebras Definition Let A be a finite flat -algebra and A be an -algebra formally of finite type and formally smooth over. We say that a surjection A A of -algebras is an embedding if it induces an isomorphism A/m A A/m A. 2. We define Emb OK to be the category whose objects and morphisms are as follows. An object of Emb OK is a triple (A A) where: A is a finite flat -algebra. 2

3 A is an -algebra formally of finite type and formally smooth over. A A is an embedding. A morphism (f,f) :(A A) (B B) of Emb OK is a pair of -homomorphisms f : A B and f : A B such that the diagram A A f f B B is commutative. 3. For a finite flat -algebra A, let Emb OK (A) be the subcategory of Emb OK whose objects are of the form (A A) and morphisms are of the form (id A, f). 4. We say that a morphism (f,f) :(A A) (B B) of Emb OK is finite flat if f : A B is finite and flat and if the map B A A B is an isomorphism. If (A A) is an embedding, the A-module ˆΩ A/OK is locally free of finite rank. Lemma For a finite flat -algebra A, the category Emb OK (A) is non-empty. 2. For objects (A A) and (A A) of Emb OK (A), there exist an object (A A) and morphisms (A A) (A A) and (A A) (A A) of Emb OK (A) such that the maps A A and A A are formally smooth. 3. For a morphism f : A B of finite flat -algebras, the following conditions are equivalent. (1) The map f : A B is flat and locally of complete intersection. (2) Their exists a finite flat morphism (f,f) :(A A) (B B) of embeddings. Proof. 1. Take a finite system of generators t 1,...,t n of A over and define a surjection [T 1,...,T n ] A by T i t i. Then the formal completion A A of [T 1,...,T n ] A is an embedding. 2. Let (A A) and (A A) be embeddings. We define A A to be the projective limit lim n(a/m n A log A /m n A ) A = lim na/m n A of the formal completions of the surjections A/m n A A /m n A A/mn A. Then A A is an embedding and satisfies the required properties. 3. (1) (2). We may assume A and B are local. We take embeddings A A and B B. Replacing B B by the projective limit lim n(a/m n A B/m n B ) B/m n B of the formal completion (A/m n A B/m n B ) B/m n B of the surjections A/mn A B/m n B B/mn B,we may assume that there is a map (A A) (B B) such that A B is formally smooth. Since A B is locally of complete intersection, the kernel of the surjection B A A B is generated by a regular sequence (t 1,...,t n ). Take a lifting ( t 1,... t n )inband define a map A[[T 1,...,T n ]] B by T i t i. We consider an embedding A[[T 1,...,T n ]] A defined by the composition A[[T 1,...,T n ]] A A sending T i to 0. Replacing A by A[[T 1,...,T n ]], we obtain a map (A A) (B B) such that the map A A B B is an isomorphism and dim A = dim B. By Nakayama s lemma, the map A B is finite. Hence the map is A B is flat by EGA Chap 0 IV Corollaire (17.3.5) (ii). 3

4 (2) (1). Since A and B are regular, B is flat and locally of complete intersection over A. Hence B is also flat and locally of complete intersection over A. The base change of an embedding by an extension of complete discrete valuation fields is defined as follows. Lemma 1.3 Let K be a complete discrete valuation field and K K be a morphism of fields inducing a local homomorphism. Let (A A) be an object of Emb OK. We define A ˆ OK to be the projective limit lim n(a/m n A ). Then the -algebra A ˆ OK is formally of finite type and formally smooth over. The natural surjection A ˆ OK A ˆ OK defines an object (A ˆ OK A OK ) of Emb OK. Proof. The -algebra A is finite over the power series ring [[T 1,...,T n ]] for some n 0. Hence the -algebra A ˆ OK is finite over [[T 1,...,T n ]] and is formally of finite type over. The formal smoothness is clear from the definition. The rest is clear.. For an object (A A) ofemb OK, we let the object (A ˆ OK A OK )ofemb OK defined in Lemma 1.3 denoted by (A A) ˆ OK. By sending (A A) to(a A) ˆ OK, we obtain a functor ˆ OK : Emb OK Emb OK. If K is a finite extension of K, we have A ˆ OK = A OK. 1.2 Tubular neighborhoods for embbedings Let (A A) be an object of Emb OK and I be the kernel of the surjection A A. Mimicing [3] Chapter 7, for a pair of positive integers m, n > 0, we define an -algebra A m/n topologically of finite type as follows. Let A[I n /π m ] be the subring of A OK K generated by A and the elements f/π m for f I n and let A m/n be its π-adic completion. For two pairs of positive integers m, n and m,n,ifm is a multiple of m and if m /n m/n, we have an inclusion A[I n /π m ] A[I n /π m ]. It induces a continuous homomorphism A m /n A m/n. Then we have the following. Lemma 1.4 Let (A A) be an object of Emb OK and m, n > 0 be a pair of positive integers. Then, 1. The -algebra A m/n is topologically of finite type over. The tensor product A m/n K = A m/n OK K is an affinoid algebra over K. 2. The map A A m/n is continuous with respect to the m A -adic topology on A and the π-adic topology on A m/n. 3. Let m,n be another pair of positive integers and assume that m is a multiple of m and j = m /n j = m/n. Then, by the map X m/n =SpA m/n K X m /n =SpA m /n K induced by the inclusion A[I n /π m ] A[I n /π m ], the affinoid variety X m/n is identified with a rational subdomain of X m /n. 4. The affinoid variety X m/n =SpA m/n K depend only on the ratio j = m/n. The proof is similar to that of [3] Lemma Proof. 1. Since the -algebra A m/n is π-adically complete, it is sufficient to show that the quotient A[I n /π m ]/(π) is of finite type over F. Since it is finitely generated over A/(π, I n ) and A/(π, I) = A/(π) is finite over F, the assertion follows. 4

5 2. Since A/π = A/(π, I) is of finite length, a power of m A is in (π m,i n ). Since the image of (π m,i n )ina m/n is in π m A m/n, the assertion follows. 3. Take a system of generators f 1,...,f N of I n and define a surjection A[I n /π m ][T 1,...,T N ]/ (π m T i f i ) A[I n /π m ] by sending T i to f i /π m. Since it induces an isomorphism after tensoring with K, its kernel is annihilated by a power of π. T 1,...,T N /(π m T i f i ) A m/n K. A m /n K Hence it induces an isomorphism 4. Further assume m/n = m /n and put k = m /m. Let f 1,...,f N I n be a system of generators of I n as above. Then A[I n /π m ] is generated by (f 1 /π m ) k1 (f N /π m ) k N, 0 k i <k as an A[I n /π m ]-module. Hence the cokernel of the inclusion A m /n A m/n is annihilated by a power of π and the assertion follows. By Lemma 1.4.4, the integral closure A j of A m/n in the affinoid algebra A m/n OK K depends only on j = m/n. Definition 1.5 Let (A A) be an object of Emb OK and j>0 be a rational number. We define A j to be the integral closure of A m/n for j = m/n in the affinoid algebra A m/n OK K and define the j-th tubular neighborhood X j (A A) to be the affinoid variety Sp A j K. In the case A = [[T 1,...,T n ]] and the map A A = is defined by sending T i to 0, the affinoid variety X j (A A) is the n-dimensional polydisk D(0,π j ) n of center 0 and of radius π j. For each positive rational number j>0, the construction attaching the j-th tubular neighboorhood X j (A A) to an object (A A) ofemb OK defines a functor X j : Emb OK (Affinoid/K) to the category of affinoid varieties over K. Forj j, we have a natural morphism X j X j of functors. For an extension K of complete discrete valuation field K, the construction of j-th tubular neighborhoods commutes with the base change. More precisely, we have the following. Let K be a complete discrete valuation field and K K be a morphism of fields inducing a local homomorphism. Then by sending an affinoid variety Sp A K over K to the affinoid variety Sp A K ˆ K K over K, we obtain a functor ˆ K K : (Affinoid/K) (Affinoid/K ) (see [2] 9.3.6). Let e be the ramification index e K /K and j>0beapositive rational number. Then the canonical map A A ˆ OK induces an isomorphism X j (A A) ˆ K K X ej ((A A) ˆ OK ) of affinoid varieties over K. In other words, we have a commutative diagram of functors X j : Emb OK (Affinoid/K) ˆ OK ˆ K K X ej : Emb OK (Affinoid/K ). Lemma 1.6 For a rational number j>0, the affinoid algebra A j K is smooth over K. Proof. By the commutative diagram above, it is sufficient to show that there is a finite separable extension K of K such that the base change X j (A A) K K = X j (A OK A OK ) 5

6 is smooth over K. Replacing K by K and separating the factors of A, we may assume A/m A = F. Then we also have A/m A = F and an isomorphism [[T 1,...,T n ]] A. We define an object (A )ofemb OK by sending T i A to 0. Let I and I be the kernel of A A and A respectively and put j = m/n. Since A/(π m,i n ) is of finite length, there is an integer n > 0 such that I n (π m,i n ). Then we have an inclusion A[I n /π m ] A[I n /π m ] and hence a map X m/n (A A) X m/n (A ). By the similar argument as in the proof of Lemma 1.4.3, the affinoid variety X m/n (A A) is identified with a rational subdomain of X m/n (A ). Since the affinoid variety X m/n (A ) is a polydisk, the assertion follows. By Lemma 1.6, the j-th tubular neighborhoods in fact define a functor X j : Emb OK (smoooth Affinoid/K) to the category of smooth affinoid varieties over K. Also by Lemma 1.6, ˆΩ A j / K is a locally free A j K -module. An idea behind the definition of the j-th tubular neighborhood is the following description of the valued points. Let (A A) be an object of Emb OK and j > 0 be a rational number. Let A j K be the affinoid algebra defining the affinoid variety Xj (A A) and let X j (A A)( K) be the set of K-valued points. Since a continuous homomorphism A jk K is determined by the induced map A O K, we have a natural injection X j (A A)( K) Hom cont.ok -alg(a,o K). For a rational number j>0, let m j denote the ideal m j = {x K; ordx j}. We regard the set Hom OK -alg(a, O K/m j )of -algebra homomorphisms as a subset of the set Hom cont.ok -alg(a,o K/m j ) of continuous -algebra homomorphisms. Lemma 1.7 Let (A A) be an object of Emb OK and j>0be a rational number. Then by the injection X j (A A)( K) Hom cont.ok -alg(a,o K) above, the set X j (A A)( K) is identified with the inverse image of the subset Hom OK -alg(a, O K/m j ) by the projection Hom cont.ok -alg(a,o K) Hom cont.ok -alg(a,o K/m j ). In other words, we have a cartesian diagram X j (A A)( K) Hom cont.ok -alg(a,o K) Hom OK -alg(a, O K/m j ) Hom cont.ok -alg(a,o K/m j ). Proof. Let j = m/n. By the definition of A m/n, a continuous morphism A O K is extended to A j K K, if and only if the image of I n is contained in the ideal (π m ). Hence the assertion follows. For an affinoid variety X over K, let π 0 (X K) denote the set lim K /Kπ 0 (X K ) of geometric connected components, where K runs over finite extensions of K in K. The set π 0 (X K) is finite and carries a natural continuous right action of the absolute Galois group G K. To get a left action, we let σ G K act on X K by σ 1. Let G K -(Finite Sets) denote the category of finite sets with a continuous left action of G K and let (Finite Flat/ ) be the category of finite flat -algebras. Then, for a rational number j>0, we obtain a sequence of functors (Finite Flat/ ) Emb OK X j (smooth Affinoid/K) X π 0 (X K) G K -(Finite Sets). 6

7 We show that the composition Emb OK G K -(Finite Sets) induces a functor (Finite Flat/ ) G K -(Finite Sets). Lemma 1.8 Let j>0 be a positive rational number. 1. Let (id A, f) :(A A) (A A) be a map of embeddings such that f : A A is formally smooth of relative dimension n. Then, we have an isomorphism X j (A A) X j (A A) D(0,π j ) n. 2. Let (A A) (B B) be a finite and flat morphism in Emb OK. Then, the induced map X j (B B) X j (A A) is a finite flat map of affinoid varieties. Proof. 1. By the assumption that the map f : A A is formally smooth, the map A A is extended to a map A A. Hence there is an isomorphism A[[T 1,...,T n ]] A such that the compostions A[[T 1,...,T n ]] A A sends T i to 0. Thus the assertion follows. 2. Let I and J = IB be the kernels of the surjections A A and B B. Since the map A B is flat, it induces isomorphisms B A A[I n /π m ] B[J n /π m ] and B A A j K Bj K. The assertion follows from this immediately. Corollary 1.9 Let (f,f) :(A A) (B B) be a morphism of Emb OK and j > 0 be a rational number. Then, 1. The induced map π 0 (X j (B B) K) π 0 (X j (A A) K) of the sets of geometric connected components depends only on f. 2. If f is an isomorphism, the map f : π 0 (X j (B B) K) π 0 (X j (A A) K) is an isomorphism. Proof. The fibers of the map Hom cont.ok -alg(a,o K) Hom cont.ok -alg(a,o K/m j ) are K-valued points of polydisks. Hence the surjection X j (A A)( K) Hom cont.ok -alg(a, O K/m j ) induces a surjection Hom cont.ok -alg(a, O K/m j ) π 0 (X j (A A) K)) by Lemma 1.7. Thus, for a morphism (f,f) :(A A) (B B), we have a commutative diagram f Hom cont.ok -alg(b,o K/m j ) Hom cont.ok -alg(a, O K/m j ) π 0 (X j (B B) K) π 0 (X j (A A) K). Hence the assertion follows. 2. It follows from Lemmas 1.2 and immediately. For a rational number j>0and a finite flat -algebra A, we put Ψ j (A) = lim (A A) Emb OK (A) π 0 (X j (A A) K). Then by Lemma and Corollary 1.9, the projective system in the right is constant. Further by Corollary 1.9.1, we obtain a functor Ψ j : Emb OK G K -(Finite Sets) sending a finite flat -algebra A to Ψ j (A). 7

8 1.3 Stable normalized integral models and their closed fibers We briefly recall the stable normalized integral model of an affinoid variety and its closed fiber (cf. [1] Section 4). It is based on the finiteness theorem of Grauert-Remmert. Theorem 1.10 (Finiteness theorem of Grauert-Remmert, [1] Theorem 4.2) Let A be an - algebra topologically of finite type. Assume that the generic fiber A K = A OK K is geometrically reduced. Then, 1. There exists a finite separable extension K of K such that the geometric closed fiber A OK OK F of the integral closure A OK of A in A OK K is reduced. 2. Assume further that A is flat over and that the geometric closed fiber A OK F is reduced. Let K be an extension of complete discrete valuation field over K and π be a prime element of K. Then the π -adic completion of the base change A OK is integrally closed in A OK K. Let A be an -algebra topologically of finite type such that A K is smooth. If a finite separable extension K satisfies the condition in Theorem , we say that the integral closure A OK of A in A K is a stable normalized integral model of the affinoid variety X K =SpA K and that the stable normalized integral model is defined over K. The geometric closed fiber X =SpecA OK OK F of a stable normalized integral model is independent of the choice of an extension K over which a stable normalized integral model is defined, by Theorem Hence, the scheme X carries a natural continuous action of the absolute Galois group G K = Gal( K/K) compatible with its action on F. The construction above defines a functor as follows. Let G K -(Aff/ F ) denote the category of affine schemes of finite type over F with a semi-linear continuous action of the absolute Galois group G K. More precisely, an object is an affine scheme Y over F with an action of G K compatible with the action of G K on F satisfying the following property: There exist a finite Galois extension K of K in K, an affine scheme Y K of finite type over the residue field F of K, an action of Gal(K /K) ony K compatible with the action of Gal(K /K) onf and a G K -equivariant isomorphism Y K F F Y. Then Theorem 1.10 implies that the geometric closed fiber of a stable normalized integral model defines a functor (smooth Affinoid/K) G K -(Aff/ F ):X X. Corollary 1.11 Let A be an -algebra topologically of finite type such that the generic fiber A K is geometrically reduced as in Theorem Let X K =SpA K be the affinoid variety and X F be the geometric closed fiber of the stable normalized integral model. Then the natural map π 0 (X F ) π 0 (X K) is a bijection. Proof. Let K be a finite separable extension of K in K such that the stable normalized integral model A OK is defined over K. Then since A OK is π-adically complete, the canonical maps π 0 (SpecA OK ) π 0 (Spec(A OK OK F )) is bijective. Since the idempotents of A K are in A OK, the canonical maps π 0 (SpecA OK ) π 0 (SpecA K ) is also bijective. By taking the limit, the assertion follows. 8

9 By Corollary 1.11, the functor (smooth Affinoid/K) G K -(Finite Sets) sending a smooth affinoid variety X to π 0 (X K) may be also regarded as the composition of the functors (smooth Affinoid/K) X X G K -(Aff/ F ) π 0 GK -(Finite Sets). For later use in the proof of the commutativity in the logarithmic case, we give a more formal description of the functor (smooth Affinoid/K) G K -(Aff/ F ):X X. For this purpose, we introduce a category lim K /K(Aff/F ) and an equivalence lim K /K(Aff/F ) G K -(Aff/ F )of categories. More generally, we define a category lim K /KV(K ) in the following setting. Suppose we are given a category V(K ) for each finite separable extension K of K and a functor f : V(K ) V(K ) for each morphism f : K K of finite separable extension of K satisfying (f g) = g f and id K =id V(K ). In the application here, we will take V(K ) to be (Aff/F ) for the residue field F. In Section 4, we will take V(K )tobeemb OK. We say that a full subcategory C of the category (Ext/K) of finite separable extensions is cofinal if C is non empty and a finite extension K of an extension K in C is also in C. We define lim K /KV(K )to be the category whose objects and morphisms are as follows. An object of lim K /KV(K )isa system ((X K ) K ob(c), (ϕ f ) f:k K mor(c)) where C is some cofinal full subcategory of (Ext/K), X K is an object of V(K ) for each object K in C and ϕ f : X K f (X K ) is an isomorphism in V(K ) for each morphism f : K K in C satisfying ϕ f f = f (ϕ f ) ϕ f for morphisms f : K K and f : K K in C. For objects X =((X K ) K ob(c), (ϕ f ) f:k K mor(c)) and Y =((Y K ) K ob(c ), (ψ f ) f:k K mor(c )) of the category lim K /KV(K ), a morphism g : X Y is a system (g K ) K ob(c ), where C is some cofinal full subcategory of C C and g K : X K Y K is a morphism in V(K ) such that the diagram X K ϕ f g K Y K ψ f f X K g K f Y K is commutative for each morphism f : K K in C. Applying the general construction above, we define a category lim K /K(Aff/F ). An equivalence lim K /K(Aff/F ) G K -(Aff/ F ) of categories is defined as follows. Let X =((X K ) K ob(c), (f ) f:k K mor(c)) be an object of lim K /K(Aff/F ). Let C K be the category of finite extensions of K in K which are in C. Then, X K = lim K C KX K is an affine scheme over F and has a natural continuous semi-linear action of the Galois group G K. By sending X to X K, we obtain a functor lim /K(Aff/F ) G K K -(Aff/ F ). We can easily verify that this functor gives an equivalence of categories. The reduced geometic closed fiber defines a functor (smooth Affinoid/K) lim K /K(Aff/F ) as follows. Let X be a smooth affinoid variety over K. Let C X be the full subcategory of (Ext/K) consisting of finite extensions K such that a stable normalized integral model A OK is defined over K. By Theorem , the subcategory C X is cofinal. Further, by Theorem , the system X = (Spec A OK OK F ) K obc X defines an object of lim K /K(Aff/F ). Thus, by sending X to X, we obtain a functor (smooth Affinoid/K) lim K /K(Aff/ F ). By taking the composition with the equivalence of categories, we recover the functor (smooth Affinoid/K) G K -(Aff/ F ). 9

10 1.4 Twisted normal cones Let (A A) be an object in Emb OK and j > 0 be a positive rational number. We define X j (A A) to be the geometric closed fiber of the stable normalized integral model of X j (A A). We will also define a twisted normal cone C j (A A) as a scheme over A F,red =(A OK F )red and a canonical map X j (A A) C j (A A). Let I be the kernel of the surjection A A. Then the normal cone C A/A of Spec A in Spec A is defined to be the spectrum of the graded A-algebra n=0 In /I n+1. We say that a surjection R R of Noetherian rings is regular if the immersion SpecR SpecR is a regular immersion. If the surjection A A is regular, the conormal sheaf N A/A = I/I 2 is locally free and the normal cone C A/A is equal to the normal bundle, namely the covariant vector bundle over SpecA defined by the locally free A-module Hom A (N A/A,A). For a rational number j, let m j be the fractional ideal m j = {x O K; ord(x) j} and put N j = m j O K F. Definition 1.12 Let (A A) be an object of Emb OK and j>0be a rational number. We define the j-th twisted normal cone C j (A A) to be the reduced part ( ) Spec (I n /I n+1 OK N jn ) of the spectrum of the A OK F -algebra n=0 (In /I n+1 OK N jn ). n=0 It is a reduced affine scheme over Spec A F,red non-canonically isomorphic to the reduced part of the base change N A/A OK F. It has a natural continuous semi-linear action of GK via N jn. The restriction to the wild inertia subgroup P is trivial and the G K -action induces an action of the tame quotient G tame K = G K /P. If the surjection A A is regular, the scheme C j (A A) is the covariant vector bundle over Spec A F,red defined by the A F,red -module (Hom A (I/I 2,A) OK N j ) A OK F A F,red. A canonical map X j (A A) C j (A A) is defined as follows. Let K be a finite separable extension of K such that the stable normalized integral model A j is defined over K and that the product je with the ramification index e = e K /K is an integer. Then, we have a natural ring homomorphism n 0 red I n OK m jen K A j : f a af. Since IA j m je K A j O, it induces a map K n In /I n+1 OK m jen K A j O /m K K Aj O. Let F K be the residue field of K. Then by extending the scalar, we obtain a map n=0 (In /I n+1 OK N jn ) A j O /m K K Aj F F. By the assumption that A j is a stable normalized integral model, we have X j (A A) = Spec (A j O /m K K Aj O K F F ). Since Xj (A A) is a reduced scheme over F, we obtain a map X j (A A) C j (A A) of schemes over F. For a positive rational number j>0, the constructions above define a functor C j : Emb OK G K -(Aff/ F ) and a morphism of functors X j C j. 10

11 Lemma 1.13 Let (A A) be an object of Emb OK and j>0be a rational number. Then, we have the following. 1. The canonical map X j (A A) C j (A A) is finite. 2. Let (A A) (B B) be a morphism in Emb OK. Then, the canonical maps form a commutative diagram X j (B B) C j (B B) Spec B F,red X j (A A) C j (A A) Spec A F,red. If the morphism (A A) (B B) is finite flat, then the right square in the commutative diagram is cartesian. 3. Assume A =. Then the surjection A A is regular and the canonical map N A/A ˆΩ A/OK A A is an isomorphism. The twisted normal cone C j (A A) is equal to the F -vector space Hom F (ˆΩ A/OK A F,N j ). The canonical map X j (A A) C j (A A) is an isomorphism. Proof. 1. Let K be a finite extension such that the stable normalized integral model A j is defined. Let A denote the π -adic completion of the image of the map n 0 In OK m jen K A OK K. Then by the definition and by Lemma 1.3, A j is the integral closure of A in A K. Hence A j O /m K K Aj is finite over n In /I n+1 OK m jen K. Thus the assertion follows. 2. Clear from the definitions. 3. If A =, there is an isomorphism [[T 1,...,T n ]] A for some n such that the composition [[T 1,...,T n ]] A maps T i to 0. Then the assertions are clear. 1.5 Étale covering of tubular neighborhoods Let A and B be the integer rings of finite étale K-algebras. For a finite flat morphism (A A) (B B) of embeddings, we study conditions for the induced finite morphism X j (A A) X j (B B) tobeétale. Let X =SpB K and Y =SpA K be affinoid varieties and A and B be the maximum integral models. Then a finite map f : X Y of affinoid varieties is uniquelly extended to a finite map A Bof integral models. Proposition 1.14 Let A and B = O L be the integer rings of finite separable extensions of K and (A A) (B B) be a finite flat morphism of embeddings. Let j>1be a rational number, π L a prime element of L and e = ordπ L be the ramification index. 1. ([1] Proposition 7.3) Assume A =. Suppose that, for each j >j, there exists a finite separable extension K of K such that the base change X j (B B) K is isomorphic to the disjoint union of finitely many copies of X j (A A) K as an affinoid variety over X j (A A). Then there is a rational number 0 <k<j such that ek is an integer and that πl ek annihilates Ω B/A. 2. ([1] Proposition 7.5) If there is a rational number 0 <k<j such that ek is an integer and that πl ek annihilates Ω B/A, then the finite flat map X j (B B) X j (A A) is étale. 11

12 Corollary 1.15 ([1] Theorem 7.2) Let A = and let B be the integer ring of a finite étale K-algebra. Let (A A) (B B) be a finite flat morphism of embeddings. Let j>1be a rational number. Suppose that, for each j >j, there exists a finite separable extension K of K such that the base change X j (B B) K is isomorphic to the disjoint union of finitely many copies of X j (A A) K as in Proposition Let I be the kernel of the surjection B B and let N B/B be the B-module I/I 2. Then, we have the following. 1. The finite map X j (B B) X j (A A) is étale and is extended to a finite étale map of stable normalized integral models. 2. The finite map X j (B B) X j (A A) is étale. 3. The twisted normal cone C j (B B) is canonically isomorphic to the covariant vector bundle defined by the B F,red-module (Hom B (N B/B,B) OK N j ) B F B F,red and the finite map X j (B B) C j (B B) is étale. Though these statements except Corollary are proved in [1] Section 7, we present here slightly modified proofs in order to compare with the proofs of the corresponding statements in the logarithmic setting given in Section 4.3. To prove Proposition 1.14, we use the following. Lemma 1.16 Let A = O L be the integer ring of a finite separable extension L, A A be an embedding and let M be an A-module of finite type. Let j>1 be a rational number and K be a finite separable extension of K such that the stable normalized integral model A j of X j (A A) is defined over K. Let e and e be the ramification indices of L and of K over K and π L and π be prime elements of L and K. Assume that e /e and e j are integers. For a rational number 0 < k < jsuch that ek is an integer, the following conditions are equivalent. (1) The A-module M = M A A is annihilated by πl ek. (2) The A j O -module K Mj = M A A j is annihilated by π e k. Proof of Lemma The image of an element in the kernel I of the surjection A A in A j is divisible by π e j. Hence we have a commutative diagram A A j A A j /(π e j ). The image of πl e A is a unit times π e in A j /(π e j ). Take a lifting a A j of the image of π L A. Since j>1and A j is π -adically complete, a e is a unit times π e in A j O. Since K Aj is normal, a is a unit times π e /e in A j O. Hence the image of π K L in A j /(π e j ) is a generator of the ideal (π e /e ). Assume that the A-module M is isomorphic to A r s i=1 A/(π L) ek i for rational numbers 0 <k 1... k s such that ek i are integers. Then the A j /(π e j )-module M j /π e j M j is isomorphic to (A j /(π e j )) r s i=1 Aj /(π min(e j,e k i ) ) since the image of π L is a generator of (π e /e )ina j /(π e j ) as shown above. The condition (1) is clearly equivalent to that r = 0 and 12

13 k i k for all i =1,...,s. We see that the condition (2) is also equivalent to this condition by taking the localization at a prime ideal A j of height 1 containing π. Proof of Proposition Since A =, there is an isomorphism [[T 1,...,T n ]] A such that the composition [[T 1,...,T n ]] A maps T i to 0. For j>0, the affinoid variety X j (A A) is a polydisk. By the proof of Lemma 1.6, there exist a finite separable extension K of K of ramification index e, an embedding (B OK B )inemb OK isomorphic to ( [[T 1,...,T n ]] N OK N ) for some N > 0, a positive rational number ɛ<jand an open immersion X j (B B) K K X e ɛ (B OK B ) as a rational subdomain. The affinoid variety X e ɛ (B OK B ) is the disjoint union of finitely many copies of polydisks. Enlarging K if necessary, we may assume that e j and e ɛ are integers. We may further assume that there is a rational number j<j <j+ ɛ such that e j is an integer, that the stable normalized integral models B j and B e ɛ of X j (B B) and of X e ɛ (B OK B ) are defined over K and that X j (B B) K is isomorphic to the disjoint union of copies of X j (A A) K. Since e j is an integer, the stable normalized integral model A j of X j (A A) is also defined over K. Then we have a commutative diagram A A j B B e ɛ B j. We consider the modules ˆΩ A/OK = lim nω (A/m n A )/ ), ˆΩ A j = lim O / K nω (A j /π n A j O )/ K as defined in the beginning of Section 1.1. By Lemma 1.4.2, we have a commutative diagram etc B j O ˆΩ K A A/OK B j O K A j B j B ˆΩ B/OK B j B e ɛ ˆΩ B e ɛ / ˆΩB j /. ˆΩ A j / By the assumption on the covering X j (B B) K X j (A A) K, the A j -algebra Bj is isomorphic to the product of finitely many copies of A j Hence the right vertical map B j A j ˆΩ A j / ˆΩ B j /. is an isomorphism. The isomorphism [[T 1,...,T n ]] A in the beginning of the proof induces an isomorphism T 1 /π e j,...,t n /π e j A j we see that the A-module ˆΩ A/OK and the A j -module ˆΩ A j / are free of rank n. Hence ˆΩ B j is also a free B j O / K -module of rank n. Further by the canonical maps Aj O ˆΩ K A A/OK ˆΩ A j, the module /O Aj K O ˆΩ K A A/OK is identified with the submodule π e j ˆΩA j the B-module ˆΩ B/OK and the B e ɛ O -module ˆΩ K B e ɛ O / K identified with the submodule π e ɛ ˆΩB e ɛ O /O. Since Xj (B B) K K is a rational subdomain of K K 13 and. Similarly, / are free of rank n and B e ɛ B ˆΩB/OK is

14 X e ɛ (B OK B ), the map B j B e ɛ ˆΩ B e ɛ / ˆΩ B j / is an injection. Combining all this, we obtain an inclusion π e j B j B ˆΩB/OK π e ɛ B j A ˆΩA/OK as submodules of ˆΩ B j. O / K Thus the B j -module Bj O K BΩ B/A = Coker(B j O ˆΩ K A A/OK B j O ˆΩ K B B/OK ) is annihilated by π e (j ɛ). Since 0 <j ɛ<j ɛ<j, it suffices to apply Lemma 1.16 (2) (1). 2. Let K be a finite separable extension such that e j is an integer and the stable normalized integral models A j and B j are defined over K. By the proof of Lemma 1.8.2, we have B j OK K = B A A j OK K and the map A j OK K B j OK K is finite flat. By Lemma 1.16 (1) (2), the B j O -module K Bj O K B Ω B/A is annihilated by π e k. Hence the map A j O K K B j O K K is étale. Proof of Corollary Here we recall the proof in [1] Section We may assume A and B are local. It follows from Proposition 1.14 that the map X j (B B) X j (A A) is finite étale. Let X j (B B) OK X j (A A) OK be the finite map of stable normalized integral models. By the assumption on the coverings X j (B B) X j (A A) for j >j, the map of integral models X j (B B) OK X j (A A) OK is étale at the origin of the closed fiber of X j (A A) OK as in the proof of [1] Theorem 7.2. Since X j (A A) OK is a regular Noetherian scheme, the assertion follows by the purity of branch locus. 2. Clear from Since the surjection B B is regular, the twisted normal cone C j (B B) is canonically isomorphic to the covariant vector bundle defined by the B F,red -module (Hom B (I/I 2,B) OK N j ) B F B F,red. We consider the commutative diagram X j (B B) C j (B B) Spec B F,red X j (A ) C j (A ) Spec F in Lemma Since the map (A A) (B B) is finite and flat, the right square is cartesian. Hence the middle vertical arrow is étale. Since A =, the lower left horizontal arrow is an isomorphism by Lemma By 2, the left vertial arrow is finite étale. Thus the assertion is proved. 2 Filtration by ramification groups: the non-logarithmic case 2.1 Construction In this subsection, we rephrase the definition of the filtration by ramification groups given in the previous paper [1] by using the construction in Section 1. The main purpose is to make clear the parallelism between the non-logarithmic construction recalled here and the logarithmic construction to be recalled in Section

15 Let Φ : (Finite Étale/K) G K-(Finite Sets) denote the fiber functor sending a finite étale K-algebra L to the finite set Φ(L) =Hom K-alg (L, K) with the continuous G K -action. In [1], we define the filtration by ramification groups on G K by constructing a family of quotient functors Φ j : (Finite Étale/K) G K-(Finite Sets),j > 0, Q of Φ satisfying a certain set of axioms. The filtration by the ramification groups G j K G K,j > 0, Q is characterized by the condition that the canonical map Φ(L) Φ j (L) induces a bijection Φ(L)/G j K Φj (L) for each finite étale algebra L over K. We recall the definition of Φ j using the notations in Section 1. The functor Φ j fits in the diagram (Finite Étale/K) Φ j G K -(Finite Sets) π 0 Ψ j (Finite Flat/ ) G K -(Aff/ F ) X X Emb OK (smooth Affinoid/K) X j We briefly recall how the other arrows in the diagram are defined. The forgetful functor Emb OK (Finite Flat/ ) sends (A A) toa. The functor X j : Emb OK (smooth Affinoid/K) is defined by the j-th tubular neighborhood. The functor (smooth Affinoid/K) G K -(Aff/ F ) sends X to the geometric closed fiber X of the stable normalized integral model. The functor π 0 : G K -(Aff/ F ) G K -(Finite Sets) is defined by the set of connected components. They induce a functor Ψ j : (Finite Flat/ ) G K -(Finite Sets). The functor Φ j is defined as the composition of Ψ j with the functor (Finite Étale/K) (Finite Flat/) sending a finite étale algebra L over K to the integral closure O L in L of. More concretely, we have Φ j (L) = lim π 0 ( X j (A O L )) (A O L ) Emb OK (O L ) for a finite étale K-algebra L. For a finite étale algebra L over K and a rational number j>0, we say that the ramification of L is bounded by j if the canonical map Φ(L) Φ j (L) is a bijection. Let A = and let B = O L be the integer ring of a finite étale K-algebra L and (A A) (B B) be a finite flat morphism of embeddings. Then, since the map X j (B B) X j (A A) is finite flat of degree [L : K], the ramification of L is bounded by j if and only if there exists a finite separable extension K of K such that the affinoid variety X j (B B) K is isomorphic to the disjoint union of finitely many copies of X j (A A) K over X j (A A) K. We say that the ramification of L is bounded by j+ if the ramification of L is bounded by every rational number j >j. The set Φ j+ (L) =Φ(L)/G j+ K for a rational number j>0has the following geometric interpretation. Let A = and let B = O L be the integer ring of a finite étale K-algebra L and (A A) (B B) be a finite flat morphism of embeddings. Let f j : X j (B B) X j (A A) and f j : Xj (B B) X j (A A) denote the canonical maps. We define the origin 0 X j (A A) to be the point corresponding to the identity map A and 0 X j (A A) to be its specialization. Then the inverse image (f j ) 1 (0) corresponds to 15

16 the set of homomorphisms B and is naturally identified with Φ(L). The set ( f j ) 1 ( 0) is also naturally identified with Φ j+ (L) and the specialization map (f j ) 1 (0) ( f j ) 1 ( 0) is then the same as Φ(L) Φ j+ (L). Consequently, if the ramification of L is bounded by j+, we may identify ( f j ) 1 ( 0) with Φ(L). Lemma 2.1 Let K K be a map of complete discrete valuation fields inducing a local homomorphism of integer rings. Assume that the a prime element of K goes to a prime element of K and that the residue field F of K is a separable extension of the residue field F of K. Then, for a rational number j>0, the map G K G K induces a surjection G j K G j K. Proof. Let A be the integer ring of a finite étale K-algebra L and (A A) be an object of Emb OK. By the assumption, the tensor product A OK is the integer ring of L K K. By the isomorphism X j (A A) ˆ K K X j (A ˆ OK A OK ) in Section 1.2 and Theorem 1.10, the natural map Φ j (L K K ) Φ j (L) is a bijection. Hence the assertion follows. Example. Let K = F p (x, y)((π)) and put L = K[t]/(t p t x ),M = L[t π p2 1,t 2 ]/(t p 1 t 1 x,t p π p3 2 t 2 y ) and G = Gal(M/K) F 3 π p3 p. Then we have Gj = G for j p 2, G j = H = Gal(M/L) F 2 p for p 2 <j p 3 and G j = 1 for p 3 <j. We put z = π p t. Then we have O L = [z]/(z p π p(p 1) z x) and L = F p (z, y)((π)). By putting s = t 1 z, we also have M = L[s, t π p2 2 ]/(s p s z( 1+πp(p 1) 2 ),t p π p(p2 p+1) 2 t 2 x ). We put π p3 M 1 = L(s) M. Then we have H j = H for j p(p 2 p + 1), H j = Gal(M/M 1 ) F p for p(p 2 p +1)<j p 3 and H j = 1 for p 3 <j. This example shows that the filtration on the subgroup H induced from the filtration by ramification groups on G is not the filtration by ramification groups on H even after renumbering. It also shows that the lower numbering filtration is not equal to the upper numbering filtration defined here even after renumbering. 2.2 Functoriality of the closed fibers of tubular neighborhoods: An equal characteristic case For a positive rational number j > 0, let (Finite Étale/K) j+ denote the full subcategory of (Finite Étale/K) consisting of étale K-algebras whose ramification is bounded by j+. In this subsection and the following one, we assume the following condition (F) is satisfied. (F) There exists a perfect subfield F 0 of F such that F is finitely generated over F 0. Further assuming that p is not a uniformizer of K, we will define a twisted tangent space Θ j and show that the functor X j : Emb OK G K -(Aff/ F ) induces a functor X j : (Finite Étale/K) j+ G K -(Finite Étale/Θj ). In this subsection, we study the easier case where K is of characteristic p. Let F 0 be a perfect subfield of F such that F is finitely generated over F 0. We assume K is of characteristic p. Then, F 0 is naturally identified with a subfield of K. We first define a functor (Finite Étale/K) Emb. In this subsection, A denotes the integer ring of a finite étale K-algebra. 16

17 Lemma 2.2 Let A be the integer ring of a finite étale K-algebra. 1. Let (A/m n A F 0 ) denote the formal completion of A/m n A F 0 of the surjection A/m n A F 0 A/m n A sending a b to ab. Then the projective limit (A ˆ F0 ) = lim n(a/m n A F 0 ) is an -algebra formally of finite type and formally smooth over. 2. Let (A ˆ F0 ) A be the limit of the surjections (A/m n A F 0 ) A/m n A. Then ((A ˆ F0 ) A) is an object of Emb OK. 3. Let A B be a morphism of the integer rings of finite étale K-algebras. Then it induces a finite flat morphism ((A ˆ F0 ) A) ((B ˆ F0 ) B) of Emb OK. Proof. 1. We may assume A is local. Let E be the residue field of A and take a transcendental basis ( t 1,..., t m )ofe over F 0 such that E is a finite separable extension of F 0 ( t 1,..., t m ). Take a lifting (t 1,...,t m )inaof ( t 1,..., t m ) and a prime element t 0 A. We define a map F 0 [T 0,...,T m ] A by sending T i to t i. Then A is finite étale over the completion of the local ring of F 0 [T 0,...,T m ] at the prime ideal (T 0 ). Hence there exist an étale scheme X over A m+1 F 0, a point ξ of X above (T 0 ) and a F 0 -isomorphism ϕ : ÔX,ξ A. Let i :SpecA X F0 be the map defined by ϕ and A. Then (A ˆ F0 ) is isomorphic to the coordinate ring of the formal completion of X F0 along the closed immersion i :SpecA X F0. Hence (A ˆ F0 ) is formally of finite type and formally smooth over. 2. Since the map (A ˆ F0 ) A is surjective, the assertion follows from Since (B ˆ F0 ) = B A (A ˆ F0 ), the assertion follows. Thus, we obtain a functor (Finite Étale/K) Emb sending a finite étale K-algebra L to ((O L ˆ F0 ) O L ). For a rational number j>0, we have a sequence of functors (Finite Étale/K) Emb (smooth Affinoid/K) G K -(Aff/ F ). We also let X j denote the composite functor (Finite Étale/K) G K-(Aff/ F ). For a finite étale K-algebra L, we have X j (L) = X j ((O L ˆ F0 ) O L ). We define an object Θ j of G K -(Aff/ F ) to be the F -vector space Θ j = Hom F (ˆΩ OK /F 0 OK F, N j ) regarded as an affine scheme over F with a natural G K -action. Let G K -(Finite Étale/Θj ) denote the subcategory of G K -(Aff/ F ) whose objects are finite étale schemes over Θ j and morphisms are over Θ j. Lemma 2.3 For a rational number j>1, the functor X j : (Finite Étale/K) G K-(Aff/ F ) induces a functor X j : (Finite Étale/K) j+ G K -(Finite Étale/Θj ). Proof. The canonical map ˆΩ OK /F 0 OK ( ˆ F0 ) ˆΩ (OK ˆ F0 ) / is an isomorphism by the definition of ( ˆ F0 ). Hence, we obtain isomorphisms X j (K) C j (( ˆ F0 ) ) Θ j by Lemma We identify X j (K) with Θ j by this isomorphism. Let L be a finite étale K-algebras whose ramification is bounded by j+. Then, by Corollary 1.15, the map X j (L) X j (K) =Θ j is finite and étale. Thus the assertion is proved. The construction in this subsection is independent of the choice of perfect subfield F 0 F by the following Lemma. 17

18 Lemma 2.4 Let K be a complete discrete valuation field of characteristic p>0 satisfying the condition (F). Let F 0 and F 0 be perfect subfields of F such that F is finitely generated over F 0 and F There exists a perfect subfield F 0 of F containing F 0 and F Assume F 0 F 0. Then F 0 is a finite separable extension of F 0. For the integer ring A of a finite étale algebra over K, the canonical map (A ˆ F0 ) (A ˆ F 0 ) is an isomorphism. Proof. 1. The maximum perfect subfield n F pn of F contains F 0 and F 0 as subfields. 2. Since F 0 is a perfect subfield of a finitely generated field F over F 0, it is a finite extension of F 0. Since the canonical map (A ˆ F0 ) (A ˆ F 0 ) is finite étale and the induced map (A ˆ F0 ) /m (A ˆ F0 ) (A ˆ F 0 ) /m (A ˆ F ) is an isomorphism, the assertion follows Functoriality of the closed fibers of tubular neighborhoods: A mixed characteristic case In this subsection, we keep the assumption: (F) There exists a perfect subfield F 0 of F such that F is finitely generated over F 0. We do not assume that the characterisic of K is p. Under the assumption (F), there exists a subfield K 0 of K such that 0 = K 0 is a complete discrete valuation ring with residue field F 0.IfK is of characteristic 0, the fraction field K 0 of the ring of the Witt vectors W (F 0 )=0 regarded as a subfield of K satisfies the conditions. If K is of characteristic p, we naturally identify F 0 as a subfield of K and the subfield F 0 ((t)) for any non-zero element t m K satisfies the conditions. In this subsection, we take a subfield K 0 of K such that 0 = K 0 is a complete discrete valuation ring with residue field F 0. Here, we do not define a functor (Finite Étale/K) Emb. Instead, we introduce a new category Emb K,OK0 and a functor Emb K,OK0 Emb OK. In this subsection, A denotes the integer ring of a finite étale K-algebra and π 0 denotes a prime element of the subfield K 0 K. For a complete Noetherian local 0 -algebra R formally smooth over 0, we define its relative dimension over 0 to be the sum tr.deg(e/k)+dim E m R /(π 0, m 2 R ) of the transcendental degree of E = R/m R over k and the dimension dim E m R /(π 0, m 2 R ). Definition 2.5 Let K be a complete discrete valuation field and K 0 be a subfield of K such that 0 = K 0 is a complete discrete valuation ring with perfect residue field F 0 and that F is finitely generated over F We define Emb K,OK0 to be the category whose objects and morphisms are as follows. An object of Emb K,OK0 is a triple (A 0 A) where: A is the integer ring of a finite étale K-algebra. A 0 is a complete semi-local Noetherian 0 -algebra formally smooth of relative dimension tr.deg(f/f 0 )+1 over 0. 18

19 A 0 A is a regular surjection of codimension 1 of 0 -algebras inducing an isomorphism A 0 /m A0 A/m A. A morphism (f,f) :(A 0 A) (B 0 B) is a pair of an -homomorphism f : A B and an 0 -homomorphism f : A 0 B 0 such that the diagram A 0 f B 0 A f B is commutative. 2. For the integer ring A of a finite étale K-algebra, we define Emb K,OK0 (A) to be the subcategory of Emb K,OK0 whose objects are of the form (A 0 A) and morphisms are of the form (id A, f). 3. We say that a morphism (A 0 A) (B 0 B) is finite flat if A 0 B 0 is finite flat and the map B 0 A0 A B is an isomorphism. Lemma If A is the integer ring of a finite étale K-algebra, then the category Emb K,OK0 (A) is non-empty. 2. Let (A 0 A) and (B 0 B) be objects of Emb K,OK0 and A B be an -homomorphism. Then there exists a homomorphism (A 0 A) (B 0 B) in Emb K,OK0 extending A B. 3. Let (A 0 A) (B 0 B) be a morphism of Emb K,OK0. If a prime element π 0 of K 0 is not a prime element of any factor of A, then the map (A 0 A) (B 0 B) is finite and flat. Proof. 1. We may assume A is local. Take a transcendental basis ( t 1,..., t m ) of the residue field E of A over k such that E is a finite separable extension of k( t 1,..., t m ). Take a lifting (t 1,...,t m )in of ( t 1,..., t m ) and a prime element t 0 of A. Then A is unramified over the completion of the local ring of 0 [T 0,...,T m ] at the prime ideal (π 0,T 0 ) by the map sending T i to t i. Hence there are an étale scheme X over A m+1 0, a point ξ of X above (π 0,T 0 ) and a regular surjection ϕ : Ô X,ξ A of codimension 1. Let A 0 be the 0 -algebra ÔX,ξ. Then (A 0 A) is an object of Emb K,OK Since A 0 is formally smooth over 0, it follows from that B 0 is the formal completion of itself with respect to the surjection B 0 B. 3. We may assume A and B are local. We show that the map B 0 A0 A B is an isomorphism. Let t 0 A 0 and t 0 B 0 be liftings of prime elements of A and B respectively, By the assumption that π 0 is not a prime element, the surjection ˆΩ A0 /0 ˆΩ A/OK0 induces an isomorphism ˆΩ A0 /0 A0 A/m A ˆΩ A/OK0 A A/m A. Hence the image of dt 0 is a basis of the kernel of ˆΩ A0 /0 A0 A/m A Ω (A/mA )/k. Therefore, (π 0,t 0 ) is a basis of m A0 /m 2 A 0. Similarly (π 0,t 0) is a basis of m B0 /m 2 B 0. Let f be a generator of the kernel of A 0 A and put f = aπ 0 + bt 0 in m A0 /m 2 A 0 for some element a, b A/m A. Since the surjection A 0 A is regular of codimension 1, either of a and b is not 0. Since the image of t 0 is a basis of m A /m 2 A and the image of f is 0, we have a 0. Hence the image of f in m B0 /m 2 B 0 is not zero. Thus the kernel of B 0 B is generated by the image of f and the map B 0 A0 A B is an isomorphism. Since B is finite over 19