A Bloch–Ogus theorem for henselian local rings in mixed characteristic

We show a conditional exactness statement for the Nisnevich Gersten complex associated to an A1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathbb {A}^1$$\end{document}-invariant cohomology theory with Nisnevich descent for smooth schemes over a Dedekind ring with only infinite residue fields. As an application we derive a Nisnevich analogue of the Bloch–Ogus theorem for étale cohomology over a henselian discrete valuation ring with infinite residue field.


Introduction
Given an A 1 -invariant cohomology theory E for smooth varieties X over a field k with Nisnevich descent, Colliot-Thélène, Hoobler and Kahn proved in [3] the exactness of the associated Gersten complex where Y = Spec(O X,x ) is the local scheme at a point x and d is the dimension of X.The main ingredient of their proof is a geometric presentation theorem [3, Theorem 3.1.1]for a closed immersion Z → X which is due to Gabber.If E is algebraic K-theory, this result implies the Gersten conjecture for smooth schemes over a field, originally proved by Quillen [7,Theorem 5.11].Taking E as étale cohomology with constant torsion coefficients defined over k, one obtains the Bloch-Ogus theorem [2].
In the mixed characteristic case of a discrete valuation ring with infinite residue field, an analogue of Gabber's geometric presentation theorem for a closed immersion Z → X was shown in [8,Theorem 2.1].However, there are two crucial differences to the equal characteristic case: Firstly, one has to require that the closed subscheme Z does not contain any irreducible component of the special fibre of X.Secondly, the presentation is not Zariski-but only Nisnevich-local in X.
In this paper, our goal is to adopt the techniques of Colliot-Thélène, Hoobler and Kahn to the mixed characteristic case using the more restricted version of the presentation theorem.Our main result is the following (see Theorem 5.12, below).
Theorem.Let S be a Dedekind scheme with only infinite residue fields and E an A 1 -invariant cohomology theory for smooth schemes of finite type over S with Nisnevich descent.Let X/S be such a smooth scheme of dimension d, x ∈ X a point and Y = Spec(O h X,x ) the Henselian local scheme at x. (1) The Gersten complex (1a) is exact possible except at the first and third (non-trivial) spot.
The authors are supported by the SFB/CRC 1085 Higher Invariants (Regensburg) funded by the DFG and the DFG-Forschergruppe 1920 Symmetrie, Geometrie und Arithmetik (Heidelberg-Darmstadt).
(2) If for each point x of X the forget support map for the special fibre Y σ RΓ Yσ (Y Nis , E) → Γ(Y, E) is trivial, then the Gersten complex is exact everywhere.
In presence of the second condition, one obtains the usual resolution of the Nisnevich sheafification of the cohomology given by E by flabby Nisnevich sheaves.If E is algebraic K-theory, the theorem was known before, see [1] and [6].
As an application of the theorem above, we derive the following analogue of the Bloch-Ogus theorem in mixed characteristic (see Corollary 6.10).
Theorem.Let Y = Spec(O h X,x ) be a Henselian local scheme of a d-dimensional smooth scheme X of finite type over a Henselian discrete valuation ring o with infinite residue field of characteristic p.Let K be a locally constant constructible sheaf of Z/m-modules for m prime to p on the small étale site of Spec(o).Then the Gersten complex is exact.Here H i (k(z), −) denotes the Galois-cohomology of the field k(z).
We remark that in [5] Geisser derived the exactness of the above Gersten complex from the Bloch-Kato-Conjecture even for a (Zariski) local scheme Y = Spec(O X,x ) but only for coefficients K = µ ⊗r m where n ≤ r.Our method of proof is more elementary, at least if the residue and quotient field of o are perfect (see Remark 6.9 and Remark 6.11).
The organization of the paper is as follows.In Section 2 we recall some known results on basechange of presheaves of spectra.We include a short reminder on elementary properties of the codimension for schemes not necessarily over a field.In Section 3 we define the coniveau filtration for a spectrum with Nisnevich descent and show that the filtration quotients are flabby Nisnevich sheaves.In Section 4 the Nisnevich Gersten complex is introduced.Up to this point, the A 1 -invariance property has not been used yet.Section 5 contains an effaceability result which makes use of A 1 -invariance and the geometric presentation theorem.This leads to our main theorem.The final Section 6 deals with the application to étale cohomology with torsion coefficients.

Preliminaries
Let S be a base scheme, which is always assumed to be noetherian and of finite dimension.Let moreover Sm S be the category of smooth schemes of finite type over S and Spt S 1 (Sm S ) the category of presheaves of spectra on Sm S .For an object X ∈ Sm S we define the category Spt S 1 (X Nis ) analogously where X Nis denotes the small Nisnevich site on X.
We consider the (stable) object-wise model structure on Spt S 1 (Sm S ).Its homotopy category SH S 1 (Sm S ) is a triangulated category with exact triangles given by the homotopy (co)fibre sequences.The left Bousfield localization at the equivalences on Nisnevich stalks is called the (stable) Nisnevich local model structure with a fibrant replacement functor L Nis .Likewise, in the case of the small site X Nis , we define the (stable) Nisnevich local model structure on Spt S 1 (X Nis ) analogously.In the case of the big site Sm S , a further left Bousfield localization yields the (stable) A 1 -Nisnevich local model structure with a fibrant replacement functor L mot .The details of this model structures play no essential role for this text and we refer to the preliminary section of [8] for further explanation and references.We are working in the non-localized model structure only and use the fibrant replacement functors to obtain statements about the localizations.Hence, whenever we speak of an exact triangle or a homotopy cofibre, we mean the respective terms for the object-wise model structure.
for each of the model structures from above (see again the preliminary section of [8] for more details and references).By abuse of notation, we write f * := f * and with the left adjoint given by post-composition.Again, precomposition with f is the right adjoint of a Quillen adjunction for the object-wise and the Nisnevich local model structure where again for the first one we have to assume that f is an object of Y Nis whereas the second always exists.
For an object X → Y in Sm Y , there is a canonical covering preserving inclusion functor δ X/Y : X Nis → Sm Y .Precomposition with this functor yields the right adjoint of a Quillen adjunction for the object-wise and the Nisnevich local model structures.The inclusion functor δ X/Y factorizes as where we set δ X := δ X/X ) and the adjunction (2b) factorizes as of Quillen adjunctions with diagonal (2b).In particular, for an étale morphism f : X → Y the restriction δ * to the respective small sites commutes with f * .In particular, these observations imply the following lemma.
Remark 2.2.Let f : X → Y be any morphism between noetherian schemes of finite Krull dimension.A diagram analogous to (2c) with f in place of f shows that the restriction δ * to the respective small sites commutes with f * .
Lemma 2.4.Let E ∈ Spt S 1 (X Nis ) be a spectrum, z ∈ X a point and consider the canonical morphism z : Spec(O X,z ) → X.Then the canonical morphism is an isomorphism of presheaves on X Nis for every n ∈ Z.
Proof.For any morphism f , we have an isomorphism f * π n (E) ∼ = π n (f * E) as π n and f * are defined object-wise.Suppose for a moment that z were an object f : U → X of the site X Nis .In this case f * (F ) ∼ = F × X U .Since homotopy presheaves and pullbacks of presheaves are calculated object-wise, we obtain We may write π n (F ) alternatively as π 0 (Hom(S n , F )), where Hom denotes the internal mapping space of preshaves.For this internal mapping space, there are isomorphisms where for the first we used that f was assumed to be in X Nis .Alltogether, we have in the case of f being an object of the site X Nis .
For the case of the essentially open immersion z : Spec(O X,z ) → X of the lemma, we write z as the cofiltered limit of the diagram D (−) : I → X Nis given by the affine Zariski neighbourhoods of z in X.Then by the proof of [8, Lemma 1.5] (and after choosing a cofinal subdiagram) one has a canonical natural isomorphism where d i : D i → X is the structural morphism which is an open immersion.The result now follows from the case handled above and from the fact that homotopy groups commute with filtered colimits.

Codimension.
In this subsection, we recall basic notations on the codimension for the convenience of the reader.Let X be a scheme and Z ⊆ X an irreducible closed subset.Define codim(Z, X) For an arbitrary closed subset Z ⊆ X we set codim(Z, X) where by convention codim(∅, X) = ∞, as the codimension of Z in X.One has If Z is irreducible closed with generic point η Z , then codim(Z, X) = dim(O X,η Z ).
Recall that a scheme X is called catenary, if for every two irreducible closed subsets Z ⊆ Z ⊆ X every maximal chain Z = Z s . . .Z 0 = Z of irreducible closed subsets has the same finite length.Examples of such are schemes (locally) of finite type over a field or over a one dimensional noetherian domain, e.g., a discrete valuation ring.
We have the following two easy lemmas.
Lemma 2.5.Let X be a scheme and Z ⊆ Z ⊆ X two irreducible closed subsets with codim(Z , X) = s.Then Lemma 2.6.Let X be a scheme and Z 1 , Z 2 ⊆ X two closed subsets with both codim(Z 1 , X) ≥ s and codim(Z 2 , X) ≥ s.Then codim(Z 1 ∪ Z 2 , X) ≥ s.
Lemma 2.7.Let X be an irreducible catenary scheme and Z ⊆ Z ⊆ X two irreducible closed subsets.Then For an integer s ≥ 0, define and say that z has codimension s in X if z ∈ X (s) .Note, that {z} is always an irreducible closed subset of X and z is its generic point.One has X (s) = ∅ for s > dim(X) as dim(X) = sup x∈X dim(O X,x ).We have codim(Z, X) = 0 if and only if Z contains a whole irreducible component of X. Hence a point z ∈ X is a generic point of an irreducible component of X if and only if dim(O X,z ) = 0. Thus, X (0) is precisely the set of generic points of irreducible components of X.
Remark 2.8.Please note that the inequality is not always an equality, even for catenary schemes.

3.1.
In this section we define the coniveau filtration for a Nisnevich local fibrant spectrum on Sm S .We fix a spectrum E ∈ Spt S 1 (Sm S ).
in Spt S 1 (X Nis ) whose homotopy fibre, we denote by Remark 3.4.Throughout Sections 3 and 4, we could as well work with arbitrary spectra E X ∈ Spt S 1 (X Nis ), not necessarily of the form δ X/S, * E for a spectrum E ∈ Spt S 1 (Sm S ).In this case, E X will be defined as f * E X for an étale morphism f : X → X of finite type.

3.5.
We fix an étale morphism f : X → X of finite type and a closed subset Z ⊆ X.Let Z = Z × X X denote the pullback of Z along f .We get an induced morphism f : Lemma 3.6.In the situation of 3.5, we have f * E Z/X E Z/ X in Spt S 1 ( XNis ).
Proof.Let j : (X Z) → X and j : ( X Z) → X.First, we apply the homotopy exact functor f * to the homotopy fibre sequence of objects from Spt S 1 (X Nis ), where j : (X Z) → X.
Proof.Note first that j : (X Z ) → X factorizes as and therefore the unit id → k * k * of the adjunction k * k * induces a morphism j * j * → j * k * k * j * j * j * .This map is compatible with the units of the adjunctions j * j * and j * j * , thus inducing the forget support map E Z/X → E Z /X on homotopy fibres.
For the exact triangle consider the diagram Applying the exact functor j * to the exact triangle yields the right vertical exact triangle where we used the definition j * E X E (X Z) .
Remark 3.8.In particular, the previous Lemma 3.7 induces in particular a long exact sequence is a pullback square such that j is an open immersion, f is an étale morphism of finite type and (X − j(U )) red × X X → (X − j(U )) red is an isomorphism.

3.10.
Recall moreover, that an object-wise fibrant spectrum E ∈ Spt S 1 (Sm S ) (i.e., every evaluation E(X) is an ordinary omega spectrum) is Nisnevich local fibrant if and only if E(∅) = * and for each Nisnevich distinguished square Q as in 3.9, the square E(Q) is a homotopy pullback square (or equivalently a homotopy pushout square).Equivalently, the sequence is a distinguished triangle and hence induces long exact sequences on homotopy groups.The same observation holds for the Nisnevich local fibrant objects of Spt S 1 (X Nis ) and the right adjoint δ X/S, * : Spt S 1 (Sm S ) → Spt S 1 (X Nis ) of the adjunction (2b) preserves Nisnevich local fibrant objects.
Lemma 3.11.An object-wise fibrant spectrum E ∈ Spt S 1 (Sm S ) is Nisnevich local fibrant if and only if for all Nisnevich distinguished squares as in 3.9, the induced morphism Proof.This follows immediately from the fact the a square of spectra is a homotopy pullback square if and only if the homotopy fibres of the horizontal morphisms are equivalent.
Lemma 3.12.Let E ∈ Spt S 1 (Sm S ) be a Nisnevich local fibrant spectrum.Let Z 1 and Z 2 be closed subsets of X, and set Then the forget support maps Denote by j : X Z → X and j i : is a homotopy pullback square.Mapping into this square from the square with edges id E X , which is a homotopy pullback square for trivial reasons, and taking homotopy fibres, yields the square (3b).Thus, (3b) is a homotopy pullback square, too.
Corollary 3.13.Let E ∈ Spt S 1 (Sm S ) be a Nisnevich local fibrant spectrum and Z 1 , . . ., Z r ⊆ X be disjoint closed subsets.Then Definition 3.14.Let E X ∈ Spt S 1 (X Nis ) be a spectrum.For an integer s ≥ 0, we define the spectrum in Spt S 1 (X Nis ).The structure maps for the colimit are the forget support maps (see Lemma 3.7).
Remark 3.15.Informally, one should think of the colimit in the previous Definition 3.14 as "making the Z's bigger".The index category is filtered as one can take the union of two closed sets.
of presheaves of spectra on X Nis .For the last equivalence, observe that the colimit in Definition 3.14 has a terminal object Z = X in the case s = 0. Definition 3.17.We denote the homotopy cofibre of E X (s+1) → E X (s) by E X (s/s+1) .

3.18.
As usual, one can associate a spectral sequence (more precisely, a presheaf on X Nis of spectral sequences) to such a situation: Applying π n for an integer n to the filtration of 3.16 yields a finite filtration and the associated spectral sequence ) is degenerate (and hence always converges in the strongest sense) as the filtration above is bounded.Reindexing and rephrasing along Definition 2.3, we get The constructed spectral sequence is not yet the coniveau spectral sequence.To obtain the latter, we will sheafify the whole situation (after taking homotopy groups as above) and identify the homotopy cofibres E X (p/p+1) with certain coproducts.Proposition 3.19.Let E ∈ Spt S 1 (Sm S ) be a Nisnevich local fibrant spectrum.Then for every integer s ≥ 0, we have an equivalence Spt S 1 (Spec(O X,z ) Nis ) : z * is the adjunction (2a) for the canonical morphism z : Spec(O X,z ) → X and where we set Z := {z} in each summand by abuse of notation.
Proof.For closed subsets Z and Z of X with Z ⊆ Z , Lemma 3.7 yields an exact triangle Taking filtered colimits yields an exact triangle of objects from Spt S 1 (X Nis ).In particular, the right-hand side is equivalent to E X (s/s+1) .The colimit in (3c) runs over the filtered category of pairs (Z, Z ) where Z ⊆ Z for Z, Z ⊆ X closed subsets of the indicated codimensions with an arrow (Z, Z ) → ( Ẑ, Ẑ ) if and only if both Z ⊆ Ẑ and Z ⊆ Ẑ .We will now rewrite this colimit.Fix a pair (Z, Z ).Since X is noetherian, Z is noetherian as a topological space and hence the union of its finite number of irreducible components Z 1 , . . ., Z r , each of codimension ≥ s.It follows, that all the intersections Z i ∩ Z j for i = j are of codimension ≥ s + 1 by Lemma 2.5.Set By Lemma 2.6, Ẑ has codimension ≥ s + 1 and ( Ẑ, Ẑ := Z ) receives a map from our original pair (Z, Z ).Let T ⊆ X (s) be the set of generic points of those Z i of codimension s.Then T ∪ Ẑ = Ẑ .Further, Û = X Ẑ ⊆ X is an open separating neighbourhood of T .By this we mean that T ∩ Û splits into a disjoint union of closures of points of T in Û .Combining these observations, we get a cofinal functor from the category of pairs (T, U ) with T ⊆ X (s) a finite subset and U ⊆ X an open separating neighbourhood of T into our original index category by mapping a pair (T, U ) to the pair (X U, T ∪ (X U )).In particular, colim As U is a separating neighbourhood of T , Corollary 3.13 gives a splitting Note that the open separating neighbourhoods of T are cofinal in all open neighbourhoods of T .In particular, we get colim Finally, by Lemma 3.6, E ({z}∩U )/U j * E {z}/X and the claim follows.
Corollary 3.20.Let E ∈ Spt S 1 (Sm S ) be a Nisnevich local fibrant spectrum.Then for every integer s ≥ 0 and every integer q, we have an isomorphism Recall that a sheaf F of abelian groups on the site X Nis is called flabby, if the presheaf H q (−, F ) on X Nis is zero for q = 0.A flabby sheaf is in particular acyclic, i.e., H q (X, F ) = 0 for q = 0. Proposition 3.22.Let E ∈ Spt S 1 (Sm S ) be a Nisnevich local fibrant spectrum, z ∈ X (s) with Z := {z} and q an integer.Then the presheaf of abelian groups is a flabby sheaf on X Nis .
Proof.Let q be an integer.Let V → X be étale of finite type with (set-theoretical) fibre V (z) over the point z ∈ X.For a point v ∈ V (z), we set Let us now prove the sheaf property of z * z * E q Z/X .Using Lemma 3.6, we may restrict us to Nisnevich covers V → X of X. Writing W := V × X V , we have to show that is an exact sequence.Since V → X is a Nisnevich cover, we can find a point v 0 in the fibre V (z) with residue field k(v 0 ) = k(z).In particular, by Lemma 3.11, the composition is an isomorphism, which settles the exactness at z * z * E q Z/X (X).For the exactness at z is a monomorphism for each v different from v 0 .But even the projection is an isomorphism by Lemma 3.11: Indeed, the equality k(v ) is (essentially) a Nisnevich neighbourhood.This finishes the proof of the sheaf property of z * z * E q Z/X .In order to show the flabbieness, let us first show that z * E q Z/X is flabby: Again, we have z * E q Z/X = E q z/X loc z by Lemma 3.6.Let j : U → X loc z be the open complement of the closed point z ∈ X loc z .Then j * E q z/X loc z is trivial by construction.Hence, E q z/X loc z is supported on z, i.e., E q z/X loc z = z * z * E q z/X loc z is flabby as a skyscrapersheaf.For the flabbiness of z * z * E q Z/X , we have to show that H i (V Nis , z * z * E q Z/X ) is trivial for all V → X étale of finite type and i > 0. Since z * E q Z/X is flabby, it is Rz * -acyclic, i.e., z * z * E q Z/X Rz * z * E q Z/X .In particular, we have ), but the latter group is trivial since V loc z,Nis → X loc z is étale and z * E q Z/X is flabby.
Corollary 3.23.Let E ∈ Spt S 1 (Sm S ) be a Nisnevich local fibrant spectrum.Then for every integer s ≥ 0 and every integer q, the presheaf E q X (s/s+1) is a flabby sheaf on X Nis .Proof.By Corollary 3.20, we have E q X (s/s+1) z∈X (s) z * z * E q Z/X .Here the direct sum is the direct sum of presheaves.But X Nis in noetherian (see e.g.[9, Proposition 5.2]), so the direct sum of sheaves is the direct sum of presheaves and the claim follows from Proposition 3.22.

The Nisnevich Gersten complex
Lemma 4.1.Let E ∈ Spt S 1 (Sm S ) be a spectrum and n an integer.The cofibre sequences of Definition 3.17 yield a complex of presheaves on X Nis of abelian groups The (Nisnevich) sheafification (−) ∼ of this complex is exact at the first spot (E n X ) ∼ if and only if the canonical map ) ∼ is zero and it is exact at the spot (E n+s X (s/s+1) ) ∼ for s ≥ 0 if both the canonical maps ) ∼ are zero (where the first condition is empty for s = 0).
Proof.The long exact sequences on homotopy groups associated to the cofibre sequences E X (s+1) → E X (s) → E X (s/s+1) from Definition 3.17 for s ≥ 0 yield a diagram and we define the middle horizontal sequence as indicated.This sequence is clearly a complex as the diagonal lines are complexes.The remaining statement follows immediately from sheafification (−) ∼ applied to the whole diagram.

4.2.
For a Nisnevich local fibrant spectrum E ∈ Spt S 1 (Sm S ) we can rewrite the complex of presheaves on X Nis of abelian groups from the previous Lemma 4.1 with the help of Corollary 3.20 as Definition 4.3.For every integer n, we define the Nisnevich Gersten complex G • (E, n) of E and homotopical degree n as the complex with entries for s ≥ 0 and zero otherwise.The differentials d s are defined as in 4.2.

4.4.
For every integer n, we can reformulate (4c) as a map into a complex of flabby sheaves (see Corollary 3.23) of abelian groups.

4.5.
In abuse of notation, we will just write E(X h x ) for the stalk of E at a point x of X.Here of course X h x denotes the Henselian local scheme Spec(O h X,x ).Proposition 4.6.Let E ∈ Spt S 1 (Sm S ) be a Nisnevich local fibrant spectrum and n an integer.There is a complex of sheaves on X Nis of abelian groups where all but the first entry are flabby Nisnevich sheaves.This complex is (1) exact at the first spot (E n X ) ∼ if and only if, for each point x of X and all Z ⊆ X closed with codim(Z, X) ≥ 1, the forget support map X) is trivial and (iii) for all s ≥ 0 and all Z ⊆ X closed with codim(Z, X) ≥ s + 2, there exists Z ⊆ Z ⊆ X closed with codim(Z , X) ≥ s + 1 such that the forget support map Proof.The complex is obtained by applying the sheafification functor to the complex (4c).By Corollary 3.23, all but the first entry are flabby Nisnevich sheaves.The exactness conditions are just expanded versions of (4a) and (4b).

Effaceability
5.1.In this chapter, let S be the spectrum of a Henselian discrete valuation ring o with infinite residue field F of characteristic p and quotient field k.Note that we make use of this hypothesis only from Lemma 5.8 onwards.
Construction 5.2.Let E ∈ Spt S 1 (Sm S ) be a spectrum and f : X → X a morphism in Sm S .We consider the morphism (5a) given on an étale morphism V → X of finite type as the map induced by the projection.This clearly generalizes the construction (3a) where the morphism f was assumed to be étale.Indeed, in this case we have we want to define a morphism (5c) that coincides with (5a) for Z = X.First note that the commutative diagram (5b) induces the base-change morphism Further, by adjunction, Construction 5.2 induces a map Composition with the unit yields a morphism which is seen to fit into a commutative square inducing the desired map η f by taking horizontal homotopy fibres.
be two morphisms of noetherian schemes of finite Krull dimension, Z ⊆ X a closed subset and Z 1 := Z × X X 1 , Z 2 := Z × X X 2 the respective base changes.Then we have a commutative triangle in Spt S 1 (X Nis ) of the respective morphisms (5c).
Proof.By adjointness it suffices to show the commutativity of the outer square of the diagram where j : U → X, j 1 : U 1 → X 1 and j 2 : U 2 → X 2 are the respective open complements of Z, Z 1 and Z 2 and where f 1|U : U 1 → U , f 2|U1 and (f 1 f 2 ) |U are the respective restrictions.The triangle on the left-hand side commutes as base change morphisms are compatible with composition and the commutativity of the remaining part is seen easily.

Recall, that a Nisnevich local fibrant spectrum
) is an equivalence for all X ∈ Sm S .Lemma 5.6.Let E ∈ Spt S 1 (Sm S ) be an A 1 -Nisnevich local fibrant spectrum.Let X ∈ Sm S be a scheme, Z ⊆ X a closed subset and π : A 1 X → X the projection.Then the canonical map (c.f.(5c)) Proof.By construction of the map in question as a homotopy fibre, it suffices to show that the two maps 5d) are both object-wise weak equivalences, This can be checked directly by evaluation on an object V → X of the site X Nis .
Lemma 5.7.Let E ∈ Spt S 1 (Sm S ) be an A 1 -Nisnevich local fibrant spectrum.Let X ∈ Sm S be a scheme and s : X → A 1 X a section of the projection π : A 1 X → X.Then there is a commutative diagram of weak equivalences.In particular, for another section s : X → A 1 X of the projection, the morphisms π * η s and π * η s are equal in the homotopy category.
Proof.This follows from the previous Lemmas 5.4 and 5.6.
V ) − 1 and the forget support map induces the trivial morphism in the homotopy category.
Proof.Consider the diagram where the non-vertical maps are the projections and j is the canonical open immersion.Let us first prove that the triangle (5e) commutes in the homotopy category, where s∞ : V → P 1 V is the section at infinity.Let s0 : V denote the zero-section.Since by Lemma 5.7 the morphism π * η s0 : is a weak equivalence, it suffices to show that the outer triangle of the enlarged diagram commutes.Indeed, the bottom triangle is obtained by applying π * to a commutative triangle considered in Lemma 5.4 for s0 = js 0 .By the same Lemma 5.4 applied to id = πs 0 , the right vertical composition is the identity.Hence, it suffices to show that π * η s0 = π * η s∞ holds in the homotopy category.Since the sections s0 and s∞ : V → P 1 V both factorize through the open immersion j : (and likewise for s∞ and s ∞ ).Here, π : thus π * η s0 = π * ηs∞ .Summing up, this yields the commutativity of diagram (5e).
In order to show that the morphism in question where the middle triangle is (5e).The left horizontal maps are induced by the respective forget support maps.For the right triangle, we note that s∞ : The commutativity of the square on the left-hand side is clear.The triangle on the right-hand side commutes again by Lemma 5.4.We observe that the lower horizontal line is given by π * applied to the exact triangle of Lemma 3.7.In particular, it is an exact triangle itself and therefore the composition is trivial.Finally, the left vertical arrow is a weak equivalence by the excision Lemma 3.11.Hence the morphism V in question is trivial in the homotopy category.For the assertion codim(A 1 Z , A 1 V ) = codim(Z, A 1 V )−1 we can argue component-wise on Z so we may assume that Z is irreducible.Further, we can replace A 1 V by a base change along a flat morphism V → V .In particular, we may assume that V is a local scheme with closed point Z.As Z is finite over Z, it is just a finite union of points in the curve A 1 Z .Thus, codim(Z, A 1 Z ) = 1 and the assertion follows by Lemma 2.7.Proposition 5.9.Let E ∈ Spt S 1 (Sm S ) be an A 1 -Nisnevich local fibrant spectrum.Let X ∈ Sm S , Z → X be a closed subscheme and x ∈ X be a point.If x lies in the special fibre X σ , assume that Z σ does not contain any connected components of X σ .Then, Nisnevich-locally on X around x, there exists a V ∈ Sm S , a smooth relative curve p : X → V with Z finite over V and a closed subscheme Z → X containing Z such that codim(Z , X) = codim(Z, X) − 1 and the forget support map induces the trivial morphism in the homotopy category.In particular, E Z/X (X) → E Z /X (X) is trivial in this case.
Proof.Possibly after shrinking X Nisnevich-locally around x, we find a Nisnevich distinguished square (5f) − → V denote the composition and set Z := p(Z) red and Z := p −1 ( Z).Since f and π and hence the composition p is flat, the assertion about the codimensions holds true.
By the excision Lemma 3.11, the upper horizontal morphism of the diagram is an equivalence, where the vertical maps are the respective forget support maps and f −1 f (Z) = Z.Application of π * yields the commutative diagram The left vertical morphism is trivial by the previous Lemma 5.8.Hence the right vertical morphism is trivial which proves the claim.

Denote by X h
x,η the generic fibre Spec(O h X,x ⊗ o k) of the Henselian local scheme at x. Similar to 4.5, by E(X h x,η ) we mean colim (W,w) E(W η ), where (W, w) runs through the Nisnevich neighbourhoods of x and W η is the generic fibre.
Corollary 5.11.Under the assumptions of Proposition 5.9, the forget support map Proof.By [8, Theorem 2.1], there is a cofinal family of Nisnevich neighbourhoods (W, w) of x admitting a Nisnevich distinguished square of the form (5f) with the additional finiteness assumption.We even claim that for such neighbourhoods (W, w), the forget support map E Z/X (W η ) → E X (W η ) is trivial.To show this, we may assume W = X, i.e., we assume X admits a Nisnevich distinguished square as in (5f) with Z/V finite.On the generic fibres, we still have a distinguished square Vη fη and as pullback, Z η /V η is still finite.Accordingly, the arguments in the proof of Proposition 5.9 go through for Z η ⊆ X η , as well.In particular, the forget support map E Z/X (X η ) → E X (X η ) is indeed trivial.
Theorem 5.12.Let S be a Dedekind scheme with only infinite residue fields.Moreover, let E ∈ Spt S 1 (Sm S ) be an A 1 -Nisnevich local fibrant spectrum and X ∈ Sm S of dimension d.The complex is exact, possible except at the spots (E n X ) ∼ and z∈X (1) z * z * E n+1 Z/X .Moreover, if for each point x of X the forget support map for the special fibre is trivial, then it is exact everywhere and thus a resolution of (E n X ) ∼ by flabby Nisnevich sheaves.In this case, we have Proof.Since exactness is checked stalk-wise and we can compute the stalk at a point x ∈ X after henselization of the local scheme obtained from S at the image of x, we may assume, that S is the spectrum of a Henselian discrete valuation ring with infinite residue field.Now the first result follows from Proposition 4.6 and Proposition 5.9.Suppose the forget support maps E Xσ/X (X h x ) → E X (X h x ) are trivial for all points x.By our assumtion and Propositions 4.6 and 5.9, it is enough so show that the forget support map x ) is trivial for closed subsets X σ Z X.We may replace X by the Henselian local scheme X h x .Write Z = Z 1 ∪ Z 2 with Z 1 = X σ and X σ Z 2 .Let U = X Z and U i = X Z i be the respective open complements.Observe that U 1 = X η and U = U 2,η are just the generic fibres.Consider the exact triangles By our assumption, the forget support map in the latter triangle is trivial, so the restriction map E X (X) → E X (U 1 ) admits a retraction r 1 .By Corollary 5.11, the forget support map in the former triangle is trivial, so the restriction map E X (U 1 ) → E X (U ) admits a retraction r 2 .Set r := r 1 • r 2 : E X (U ) → E X (X).By construction, r is a retraction of the restriction map E X (X) → E X (U ).Thus, using the exact triangle we get that the forget support map E Z/X (X) → E X (X) is indeed trivial.

A Bloch-Ogus theorem for étale cohomology
In this section we want to apply Theorem 5.12 to étale cohomology.Let us first fix the situation: 6.1.We are in the situation of 5.1.For the whole section, we fix an essentially smooth scheme X/S, connected and of finite dimension.Let us denote the structural morphism by p X : X → S. We fix a coefficient group Λ := Z/m for an integer m > 0 prime to p.We work in the derived category D b c (X et , Λ) of bounded (above and below) complexes all of whose cohomology sheaves are constructible sheaves of Λ-modules.By an l.c.c.complex K • , we mean a complexes K • ∈ D b c (X et , Λ) with locally constant cohomology sheaves H q (K • ) for all q.6.2.Let ε : X et → X Nis be the canonical morphism of sites.Note that RΓ(X et , −) RΓ(X Nis , Rε * (−)).By abuse of notation, let us denote by ε also the corresponding morphism Sm S,et → Sm S,Nis of the smooth sites.For an l.c.c.complex K • in D b c (S et , Λ), we denote by K • also the complex in D b (Sm S,et , Λ) that restricts to p * X K • on each small site X et .Further, we fix a Nisnevich local fibrant spectrum [4,Corollary 7.7.4]) and hence a quasi-isomorphism on cohomology Under the Dold-Kan correspondence this translates to our claim.
6.4.In order to apply Theorem 5.12 to the A 1 -local spectrum E(K • ), we need to show that the forget support maps x ) vanish for all points x in X. Unravelling the definitions, these maps are just the forget support maps In the following, we will make use of Gabber's absolute purity theorem -but not in its full strength.The following easy special case will be sufficient for our cause: Lemma 6.5.In the situation of 6.1, let i : Z → X be a closed subscheme of codimension c, contained in the special fibre of X/S.Assume Z/F is smooth and connected.Then the canonical morphism Ri ).Further, by the special case of absolute purity for the closed point in S (which is an easy exercise -e.g. the proof of [4,Lemma 8.3.6]goes through unchanged for l.c.c.sheaves and hence for l.c.c.complexes), Rσ !K • σ * K • [−2].Summing up, we get finishing the proof.Lemma 6.6.In the situation of 6.1, assume that X is Henselian local with closed point x in the special fibre of X/S.Then the canonical morphism σ X, * Rσ !X Λ → Λ induces the trivial morphism in D b (k(x) et , Λ): In particular, the canonical map RΓ Xσ (X et , Λ) → RΓ(X et , Λ) is trivial.
Combining Theorem 5.12 and Corollary 6.7, we get: Theorem 6.8.Let S be the spectrum of a Henselian discrete valuation ring with infinite residue field F. Let X/S be smooth, d = dim(X) and K • an l.c.c.complex in D b c (S et , Λ).Then the Nisnevich Gersten complex G • (E(K • ), n) is a flasque resolution of the Nisnevich sheafification R n ε * K • | X of étale cohomology with coefficients K • .In particular, we get the exact sequence Proof.The spectrum E(K • ) is A 1 -local by Lemma 6.3.Combining Theorem 5.12 and Corollary 6.7, we get that G Remark 6.9.We can avoid absolute purity in its full strength if we assume k and F to be perfect: Computing z * E(K • ) n+s Z/X under this assumption, we may assume Z to be smooth over k (if z is contained in the generic fibre of X/S) or smooth over F (if z is contained in the special fibre of X/S) by generic smoothness.In both cases, z * E(K • ) n+s Z/X ∼ = z * H n−s (k(z), K • (−s)), either by relative purity or by Lemma 6.5.

2. 1 .
Basechange.A morphism f : X → Y of noetherian schemes of finite Krull dimension induces a covering preserving functor f : Sm Y → Sm X by pullback.Precomposition f * with f is the right adjoint of a Quillen adjunction f