# Basic results on Grothendieck Duality

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1 Basic results on Grothendieck Duality Joseph Lipman 1 Purdue University Department of Mathematics lipman November Supported in part by NSA Grant # H : Joseph Lipman (Purdue University) Grothendieck Duality November / 25

2 Outline 1 Derived direct- and inverse-image pseudofunctors 2 Global Duality 3 Tor-independent Base Change; Sheafified Duality 4 Twisted Inverse-image Pseudofunctor 5 Perfect maps and complexes 6 Dualizing Complexes 7 Comments and Problems Joseph Lipman (Purdue University) Grothendieck Duality November / 25

3 Definition (Pseudofunctor: special case of 2-functor ) A contravariant pseudofunctor on a category S assigns to each X S a category X #, to each map f : X Y a functor f # : Y # X # (with 1 # = 1), and to each map-pair X f Y g Z a functorial isomorphism d f, g : f # g # (gf ) # satisfying d 1, g = d g,1 = identity, and such that for each triple of maps X f Y g Z h W the following commutes: (hgf ) # d g f h (gf ) # h # d f,hg f # (hg) # d g,h d f, g f # g # h # Covariant pseudofunctor is similarly defined, with arrows reversed, i.e., it means contravariant functor on S op. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

4 Examples: Derived inverse-image (contravariant). Derived direct-image (covariant). S:= category of ringed spaces X # := D(X ) ( derived category of {OX -modules}) f # := Lf resp. f # := Rf Relations between Lf and Rf 1. For any ringed-space map f : X Y, Lf : D(Y ) D(X ) is left-adjoint to Rf, i.e., for E D(Y ), F D(X ), Hom D(X ) (Lf E, F ) = Hom D(Y ) (E, Rf F ). Joseph Lipman (Purdue University) Grothendieck Duality November / 25

5 Relations between Lf and Rf (ct d) 2. For any commutative square of ringed-space maps one has the functorial map natural composition g X Y v X σ u f Y θ = θ σ : Lu Rf Rg Lv, adjoint to the Rf Rf Rv Lv Ru Rg Lv. If σ is a fiber square of concentrated (= quasi-compact, quasi-separated) schemes then, with D qc the full subcategory of D whose objects are the complexes with quasi-coherent homology, θ σ is an isomorphism of functors on D qc σ is tor-independent. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

6 Grothendieck operations The adjoint pseudofunctors Rf and Lf, and the derived sheaf-hom and Tensor functors also adjoint, i.e., for any ringed-space X there is a natural isomorphism Hom D(X ) (E = X F, G) Hom D(X ) ( E, RHomX (F, G) ) are four of the six operations of Grothendieck. A fifth, right adjoint to Rf, is about to be introduced. These operations, and their interrelations, generate an incredibly rich structure, around which e.g., Grothendieck Duality is built. For examples, Sheafified adjointness of Lf and Rf Rf RHom X (Lf E, F ) = RHom Y (E, Rf F ) ( E D(Y ), F D(X ) ) Projection isomorphism for concentrated f : X Y Rf (Lf E = X F ) ( E = Y Rf F E Dqc (Y ), F D qc (X ) ) Joseph Lipman (Purdue University) Grothendieck Duality November / 25

7 2. Global Duality Theorem Grothendieck Duality begins with this theorem: Let X be a concentrated scheme and f : X Y a concentrated scheme-map. Then the -functor Rf : D qc (X ) D(Y ) has a bounded-below right -adjoint. More elaborately, For f : X Y as before, there is a bounded-below -functor (f, identity): D(Y ) D qc (X ) and a map of -functors τ : Rf f 1 such that for all F D qc (X ) and G D(Y ), the natural composite -functorial map (in the derived category of abelian groups) RHom X (F, f G ) RHom X (Lf Rf F, f G ) is a -functorial isomorphism. RHom Y (Rf F, Rf f G ) τ RHom Y (Rf F, G ) Joseph Lipman (Purdue University) Grothendieck Duality November / 25

8 Definitions (Needed for not-necessarily-noetherian situations.) Definition An O X -complex (X a scheme) is pseudo-coherent if its restriction to each affine open subscheme is D-isomorphic to a bounded-above complex of finite-rank locally free sheaves. When X is noetherian, pseudo-coherent just means: has coherent homology modules, vanishing in all large degrees. Definition A scheme-map f : X Y is quasi-proper if Rf takes pseudo-coherent O X -complexes to pseudo-coherent O Y -complexes. When X is noetherian, and f finite type and separated, quasi-proper simply means proper. For quasi-proper f we write f! in place of f. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

9 3. Tor-independent Base Change Theorem Here is the other basic building block of the theory. Theorem Suppose there is given a tor-independent fiber square g X Y v X σ f u Y 8 >< >: f (hence g) quasi-proper u of finite tor-dimension Then the functorial map adjoint to the natural composition is an isomorphism Rg Lv f! G β σ (G): Lv f! G above Lu Rf f! G Lu G, Lu τ g! Lu G ( G D + qc (Y ) ) (where G D + qc means G D qc and H n (G) = 0 for all n 0. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

10 Corollary: Sheafified Duality The Base-change Theorem for open immersions u is equivalent to the following Sheafified Duality Theorem. Theorem Let f : X Y be quasi-proper. Then for any F D qc (X ), G D + qc(y ), the composite duality map is an isomorphism. Rf RHom X (F, f G ) Rf RHom X (Lf Rf F, f G ) RHom Y (Rf F, Rf f G ) τ RHom Y (Rf F, G ) Global Duality results from this by application of the functor RΓ(Y, ). (Neeman) Theorem fails without the boundedness restriction on G. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

11 4. Twisted Inverse-image Pseudofunctor On the category S f of finite-type separated maps of noetherian schemes, a D + qc-valued pseudofunctor! that is uniquely determined up to isomorphism by the following three properties: (i) The pseudofunctor! restricts on the subcategory of proper maps to a right adjoint of the derived direct-image pseudofunctor. (ii) The pseudofunctor! restricts on the subcategory of étale maps to the (derived or not) inverse-image pseudofunctor. (iii) For any fiber square v g σ f (f, g proper; u, v étale), u the base-change map β σ : v f! g! u, adjoint to the natural composition Rg v f! u Rf f! u, above is the natural composite isomorphism v f! = v! f! (fv)! = (ug)! g! u! = g! u. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

12 Remarks 1. The proof uses Nagata s compactification theorem: Every finite-type separated map of noetherian (or just concentrated) schemes factors as proper open immersion. Use the base change theorem to paste the pseudofunctors on proper maps and on open immersions. The problem is to show that everything is independent of choice of compactification for the maps. 2. Without noetherian hypotheses, Nayak showed, without needing Nagata theorem, that there is a! as above, but over the smallest subcategory of arbitrary concentrated schemes which contains all flat finitely-presented proper maps and all separated étale maps. 3. (Something concrete.) When f is both étale and proper hence finite and flat then for any such!, the natural f f = f f! 1 is nothing but the standard trace map. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

13 Interaction of twisted inverse image with RHom For any scheme-map f : X Y there is a natural pseudofunctorial (i.e., transitive w.r.t. composition in S f ) map ψ f E,F : RHom X (Lf E, f! F ) f! RHom Y (E, F ) agreeing with the obvious one when f is étale (so that f! = f ), and dual, when f is proper, to the natural composition Rf RHom X (Lf E, f! F ) above RHom Y (E, Rf f! F ) RHom Y (E, F ). ψ f E,F is an isomorphism if E is pseudo-coherent and F D+ qc(y ). Joseph Lipman (Purdue University) Grothendieck Duality November / 25

14 Interaction of twisted inverse image with = = = For any S f -map f : X Y there is a natural functorial map, defined via compactification and the projection isomorphism (details on request) χ f E : f! O Y = Lf E f! E ( E D + qc (Y ) ) ; and, at least in the noetherian case, f is perfect (i.e., has finite tor-dimension) = χ f E and conversely when f is proper. iso for all E; Consequently (and more generally in appearance): When f is perfect there is a natural functorial isomorphism χ f E,F : f! E = Lf F f! (E = F ) ( E, F D + qc (Y ) ). Thus for perfect f the study of f! is reduced, modulo properties of =, to that of the relative dualizing complex f! O Y. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

15 5. Perfection Definitions Given a scheme-map f : X Y, we say an O X -complex E is f -perfect if E is pseudo-coherent and has finite relative tor-dimension (i.e., there are integers m n such that the stalk E x at each x X is, as an O Y,f (x) -complex, D-isomorphic to a flat complex vanishing in degrees outside [m, n]). E is perfect if X is covered by open sets over which E is D-isomorphic to a bounded complex of finite-rank free O X -modules. ( E is 1 X -perfect.) Theorem Equivalent for a finite-type separated f : X Y with X, Y, noetherian: (i) The map f is perfect, i.e., the complex O X is f -perfect. (ii) The complex f! O Y is f -perfect. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

16 continuation of Theorem (iii) f! O Y D qc(x ), and for every F D + qc(y ), the D qc (X )-map χ f O Y,F f! O Y f F f! F is an isomorphism. (iii) For every perfect O Y -complex E, f! E is f -perfect; and for all E, F D(Y ) such that E and E F are in D + qc(y ), the D qc (X )-map is an isomorphism. χ f E,F f! E f F f! (E F ). Joseph Lipman (Purdue University) Grothendieck Duality November / 25

17 Proper perfect maps For a proper map f : X Y of noetherian schemes: f is perfect Rf takes perfect O X -complexes to perfect O Y -complexes. f is perfect unrestricted tor-independent base change holds: For any tor-independent fiber square of noetherian schemes g X Y v X σ u and G D qc (Y ) the above-defined base-change map is an isomorphism f Y β σ (G): Lv f! G g! Lu G. (u need not have finite tor-dimension, and G need not be bounded below.) Joseph Lipman (Purdue University) Grothendieck Duality November / 25

18 Proper perfect maps (continued) If f is perfect and (F α ) is a small filtered direct system of flat quasi-coherent O Y -modules then for all n Z the natural map is an isomorphism lim H n (f! F α ) H n (f! lim F α ). α α Joseph Lipman (Purdue University) Grothendieck Duality November / 25

19 6. Dualizing Complexes Definition A dualizing complex R on a noetherian scheme X is a complex with coherent homology that is D-isomorphic to a bounded injective complex, and has the following equivalent properties: (i) For every F D c (X ), the map that is (derived) Hom-Tensor adjoint to the natural composition is an isomorphism F RHom(F, R) F RHom(F, R) F R RHom(RHom(F, R), R). (ii) Condition (i) holds for F = O X, i.e., the map O X RHom(R, R) which takes 1 Γ(X, O X ) to the identity map of R is an isomorphism. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

20 Remarks Grothendieck s original strategy for proving duality for proper not-necessarily-projective maps of noetherian schemes, at least for bounded-below complexes with coherent homology, is based on pseudofunctorial properties of dualizing complexes. The basic problem in this approach is the construction of a coherent family of dualizing complexes, or, roughly speaking, a pseudofunctor on proper maps with properties like those of ( )!, but taking values in dualizing complexes. Though this approach gives less general results than stated before, it is not without interest historical and otherwise; and indeed, for formal schemes, it yields results not otherwise obtainable (as of 2007), see Sastry s paper in Contemporary Math This paper is preceded by one (by Sastry, Nayak and L.) in which the original construction in Residues and Duality is simplified, and generalized to Cousin complexes on formal schemes. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

21 More Remarks For connected X, dualizing O X -complexes, if they exist, are unique up to tensoring with a shifted invertible O X -module. The existence of a dualizing complex places restrictions on X. For instance, X must be universally catenary and of finite Krull dimension. (Very recently, Neeman generalized the definition of dualizing complex to where it applies to bounded coherent complexes without the finite-dimensionality restriction on X.) Any scheme of finite type over a regular (or even Gorenstein) scheme of finite Krull dimension has a dualizing complex. The relation between dualizing complexes and the twisted inverse image pseudofunctor ( )! is rooted in the following, by now classical, Proposition. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

22 Proposition Let f : X Y be a finite-type separated map of noetherian schemes, and let R be a dualizing O Y -complex. Then with R f := f! R, and D Rf ( ):= RHom(, R f ), it holds that (i) R f is a dualizing O X -complex. (ii) There is a functorial isomorphism or equivalently, f! D R F D Rf Lf F ( F D c (Y ) ) f! E D Rf Lf D R E ( E D + c (Y ) ). (i) (ii) via the above iso ψ f E,F : RHom X (Lf E, f! F ) f! RHom Y (E, F ). This Proposition suggests how a coherent system of dualizing complexes, when such exists can give rise to a twisted inverse-image pseudofunctor. Details in Notes on Derived Functors and Grothendieck Duality, lipman Joseph Lipman (Purdue University) Grothendieck Duality November / 25

23 7. Comments and Problems Even leaving aside applications, there s lots more to Duality theory, especially enlivening concrete interpretations, e.g., via differentials and familiar maps like traces and residues. There is also generalization to formal schemes, which unifies global and local duality; and analogous theories for étale cohomology, analytic spaces, etc., etc. Mention one problem in the abstract vein: Problem Can the twisted inverse-image pseudofunctor and its basic properties (as above) be extended to the category of essentially finite-type separated maps of noetherian schemes? Probably so, but not trivially. Being essentially of finite type is a local condition, so it s not clear that useful global properties like compactifiability obtain. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

24 More comments and problems The idea is then to use Nayak s methods of pasting pseudofunctors, methods which don t require compactifiability, but do require (as do all of the preceding results!) verification of commutativity of complicated diagrams. In fact these massively time-consuming verifications take up a major (and essential) part of the above-mentioned notes, suggesting that: Problem It would be very nice if someone came up with a coherence theorem, or at least an expert computer program, to make such tedium unnecessary. A serious attempt to do this could lead to a much deeper understanding of how and why the formalism works, not to mention the potential interest of the artificial intelligence aspects. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

25 We can also ask: Problem Can the twisted inverse-image pseudofunctor and its basic properties be extended to a nonnoetherian context, or to category of essentially pseudo-finite-type separated maps of noetherian formal schemes? As already indicated, Nayak has done this to a considerable extent. Pursuing this further might give some insight into: Final (for today) Problem In some sense the twisted inverse image is too good to be true. Why do these pastings of two quite different pseudofunctors, for proper resp. étale maps, and of canonical maps of these pseudofunctors pastings which depend on complicated compatibilities, whether from the abstract or the concrete point of view work out so well? There may well be an as yet undiscovered deeper underlying structure which would explain it all. Joseph Lipman (Purdue University) Grothendieck Duality November / 25

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