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Enriched category theory
enriched category theory Backgroundcategory theory monoidal category, closed monoidal category cosmos, multicategory, bicategory, double category, virtual double category Basic conceptsenriched category enriched functor, profunctor enriched natural transformation enriched adjoint functor enriched product category enriched functor category Universal constructionsweighted limit end, coend Extra stuff, structure, propertycopowering (tensoring) powering (cotensoring) monoidal enriched category cartesian closed enriched category locally cartesian closed enriched category Homotopical enrichmentenriched homotopical category enriched model category model structure on homotopical presheaves Additive and abelian categoriesadditive and abelian categories Context and backgroundenriched category theory homological algebra stable homotopy theory CategoriesAb-enriched category pre-additive category pseudo-abelian categoryadditive category, (AB1) pre-abelian category (AB2) abelian category (AB5) Grothendieck category Functorsadditive functor left/right exact functor Derived categoriesderived category derived functor homotopy category triangulated category Edit this sidebar Homological algebrahomological algebra (also nonabelian homological algebra) Introduction Context additive and abelian categories Ab-enriched category pre-additive category additive category pre-abelian category abelian category Grothendieck category abelian sheaves semi-abelian category Basic definitions kernel, cokernel complex differential homology category of chain complexes chain complex chain map chain homotopy chain homology and cohomology quasi-isomorphism homological resolution simplicial homology generalized homology exact sequence, short exact sequence, long exact sequence, split exact sequenceinjective object, projective object injective resolution, projective resolution flat resolution Stable homotopy theory notions derived category triangulated category, enhanced triangulated category stable (∞,1)-category stable model category pretriangulated dg-category A-∞-category (∞,1)-category of chain complexes derived functor, derived functor in homological algebra Tor, Ext homotopy limit, homotopy colimit abelian sheaf cohomology Constructions double complex Koszul-Tate resolution, BRST-BV complex spectral sequence spectral sequence of a filtered complex spectral sequence of a double complex Grothendieck spectral sequence Leray spectral sequence Serre spectral sequence Hochschild-Serre spectral sequence Lemmas diagram chasing 3x3 lemma four lemma, five lemma snake lemma, connecting homomorphism horseshoe lemma Baer's criterion Schanuel's lemma Homology theories singular homology cyclic homology Theorems Dold-Kan correspondence / monoidal, operadic Moore complex, Alexander-Whitney map, Eilenberg-Zilber mapEilenberg-Zilber theorem cochain on a simplicial setuniversal coefficient theorem Künneth theorem Contents Definition Properties Related concepts References Definition DefinitionAn additive category is a category which is an Ab-enriched category; (sometimes called a pre-additive category–this means that each hom-set carries the structure of an abelian group and composition is bilinear) which admits finite coproducts (and hence, by prop. below, finite products which coincide with the coproducts, hence finite biproducts). The natural morphisms between additive categories are additive functors. RemarkA pre-abelian category is an additive category which also has kernels and cokernels. Equivalently, it is an Ab-enriched category with all finite limits and finite colimits. An especially important sort of additive category is an abelian category, which is a pre-abelian one satisfying the extra exactness property that all monomorphisms are kernels and all epimorphisms are cokernels. See at additive and abelian categories for more. RemarkThe Ab-enrichment of an additive category does not have to be given a priori. Every semiadditive category (a category with finite biproducts) is automatically enriched over commutative monoids (as described at biproduct), so an additive category may be defined as a category with finite biproducts whose hom-monoids happen to be groups. (The requirement that the hom-monoids be groups can even be stated in elementary terms without discussing enrichment at all, but to do so is not very enlightening.) Note that the entire AbAb-enriched structure follows automatically for abelian categories. RemarkSome authors use additive category to simply mean an Ab-enriched category, with no further assumptions. It can also be used to mean a CMonCMon-enriched (commutative monoid enriched) category, with or without assumptions of products. Properties PropositionIn an Ab-enriched category (or even just a CMonCMon-enriched category), a finite product is also a coproduct, and dually (hence a biproduct). This statement includes the zero-ary case: any terminal object is also an initial object, hence a zero object (and dually), hence every additive category has a zero object. More precisely, for {X i} i∈I\{X_i\}_{i \in I} a finite set of objects in an Ab-enriched category, the unique morphism ∐i∈IX i⟶∏j∈IX j \underset{i \in I}{\coprod} X_i \longrightarrow \underset{j \in I}{\prod} X_jwhose components are identities for i=ji = j and are zero otherwise is an isomorphism. ProofConsider first the nullary (i.e., zero-ary) case. Given a terminal object *\ast, the unique morphism id *:*→*id_\ast: \ast \to \ast is the zero morphism 00 in its hom-object. For any object AA, the zero morphism 0 A:*→A0_A: \ast \to A must equal any morphism f:*→Af: \ast \to A on account of f=fid *=f0=0 Af = f id_\ast = f 0 = 0_A where the last equation is by CMonCMon-enrichment. Hence *\ast is initial. (N.B.: this argument applies more generally to categories enriched in pointed sets, and is self-dual.) Consider now the case of binary (co-)products. Using zero morphisms, in addition to its canonical projection maps p i:X 1×X 2→X ip_i \colon X_1 \times X_2 \to X_i, any binary product also admits “injection” maps X i→X 1×X 2X_i \to X_1 \times X_2, and dually for the coproduct: X 1 X 2 ↘ (id,0) (0,id)↙ id X 1↓ X 1×X 2 ↓ id X 2 ↙ p X 1 p X 2↘ X 1 X 2,X 1 X 2 ↘ i X 1 i X 2↙ id X 1↓ X 1⊔X 2 ↓ id X 2 ↙ (id,0) (0,id)↘ X 1 X 2. \array{ X_1 && && X_2 \\ & \searrow^{\mathrlap{(id,0)}} && {}^{\mathllap{(0,id)}}\swarrow \\ {}^{\mathllap{id_{X_1}}}\downarrow && X_1 \times X_2 && \downarrow^{\mathrlap{id_{X_2}}} \\ & \swarrow_{\mathrlap{p_{X_1}}} && {}_{\mathllap{p_{X_2}}}\searrow \\ X_1 && && X_2 } \;\;\;\;\;\;\;\;\;\;\;\;\,,\;\;\;\;\;\;\;\;\;\;\;\; \array{ X_1 && && X_2 \\ & \searrow^{\mathrlap{i_{X_1}}} && {}^{\mathllap{i_{X_2}}}\swarrow \\ {}^{\mathllap{id_{X_1}}}\downarrow && X_1 \sqcup X_2 && \downarrow^{\mathrlap{id_{X_2}}} \\ & \swarrow_{\mathrlap{(id,0)}} && {}_{\mathllap{(0,id)}}\searrow \\ X_1 && && X_2 } \,.Observe some basic compatibility of the AbAb-enrichment with the product: First, for (α 1,β 1),(α 2,β 2):R→X 1×X 2(\alpha_1,\beta_1), (\alpha_2, \beta_2)\colon R \to X_1 \times X_2 then (⋆)(α 1,β 1)+(α 2,β 2)=(α 1+α 2,β 1+β 2) (\star) \;\;\;\;\;\; (\alpha_1,\beta_1) + (\alpha_2, \beta_2) = (\alpha_1+ \alpha_2 , \; \beta_1 + \beta_2)(using that the projections p 1p_1 and p 2p_2 are linear and by the universal property of the product). Second, (id,0)∘p 1(id,0) \circ p_1 and (0,id)∘p 2(0,id) \circ p_2 are two projections on X 1×X 2X_1\times X_2 whose sum is the identity: (⋆⋆)(id,0)∘p 1+(0,id)∘p 2=id X 1×X 2. (\star\star) \;\;\;\;\;\; (id, 0) \circ p_1 + (0, id) \circ p_2 = id_{X_1 \times X_2} \,.(We may check this, via the Yoneda lemma on generalized elements: for (α,β):R→X 1×X 2(\alpha, \beta) \colon R \to X_1\times X_2 any morphism, then (id,0)∘p 1∘(α,β)=(α,0)(id,0)\circ p_1 \circ (\alpha,\beta) = (\alpha,0) and (0,id)∘p 2∘(α,β)=(0,β)(0,id)\circ p_2\circ (\alpha,\beta) = (0,\beta), so the statement follows with equation (⋆)(\star).) Now observe that for f i:X i→Qf_i \;\colon\; X_i \to Q any two morphisms, the sum ϕ≔f 1∘p 1+f 2∘p 2:X 1×X 2⟶Q \phi \;\coloneqq\; f_1 \circ p_1 + f_2 \circ p_2 \;\colon\; X_1 \times X_2 \longrightarrow Qgives a morphism of cocones X 1 X 2 ↘ (id,0) (0,id)↙ id X 1↓ X 1×X 2 ↓ id X 2 X 1 ↓ ϕ X 2 f 1↘ ↙ f 2 Q. \array{ X_1 && && X_2 \\ & \searrow^{\mathrlap{(id,0)}} && {}^{\mathllap{(0,id)}}\swarrow \\ {}^{\mathllap{id_{X_1}}}\downarrow && X_1 \times X_2 && \downarrow^{\mathrlap{id_{X_2}}} \\ & && \\ X_1 && \downarrow^{\mathrlap{\phi}} && X_2 \\ & {}_{\mathllap{f_1}}\searrow && \swarrow_{\mathrlap{f_2}} \\ && Q } \,.Moreover, this is unique: suppose ϕ′\phi' is another morphism filling this diagram, then, by using equation (⋆⋆)(\star \star), we get ϕ =ϕ∘id X 1×X 2 =ϕ∘((id X 1,0)∘p 1+(0,id X 2)∘p 2) =ϕ∘(id X 1,0)∘p 1+ϕ∘(0,id X 2)∘p 2 =f 1∘p 1+f 2∘p 2 =ϕ′∘(id X 1,0)∘p 1+ϕ′∘(0,id X 2)∘p 2 =ϕ′∘((id X 1,0)∘p 1+(0,id X 2)∘p 2) =ϕ′∘id X 1×X 2 =ϕ′. \begin{aligned} \phi & = \phi \circ id_{X_1 \times X_2} \\ &= \phi \circ ( (id_{X_1},0) \circ p_1 + (0,id_{X_2})\circ p_2 ) \\ & = \phi \circ (id_{X_1}, 0) \circ p_1 + \phi \circ (0, id_{X_2}) \circ p_2 \\ & = f_1 \circ p_1 + f_2 \circ p_2 \\ & = \phi' \circ (id_{X_1}, 0) \circ p_1 + \phi' \circ (0, id_{X_2}) \circ p_2 \\ & = \phi' \circ ( (id_{X_1},0) \circ p_1 + (0,id_{X_2})\circ p_2 ) \\ &= \phi' \circ id_{X_1 \times X_2} \\ &= \phi' \end{aligned} \,.This means that X 1×X 2X_1\times X_2 satisfies the universal property of a coproduct. By a dual argument, the binary coproduct X 1⊔X 2X_1 \sqcup X_2 is seen to also satisfy the universal property of the binary product. By induction, this implies the statement for all finite (co-)products. (If a particular finite (co-)product exists but binary ones do not, one can adapt the above argument directly to that case.) RemarkSuch products which are also coproducts as in prop. are sometimes called biproducts or direct sums; they are absolute limits for Ab-enrichment. RemarkThe coincidence of products with biproducts in prop. does not extend to infinite products and coproducts.) In fact, an Ab-enriched category is Cauchy complete just when it is additive and moreover its idempotents split. Conversely: DefinitionA semiadditive category is a category that has all finite products which, moreover, are biproducts in that they coincide with finite coproducts as in def. . PropositionIn a semiadditive category, def. , the hom-sets acquire the structure of commutative monoids by defining the sum of two morphisms f,g:X⟶Yf,g \;\colon\; X \longrightarrow Y to be f+g≔X→Δ XX×X≃X⊕X⟶f⊕gY⊕Y≃Y⊔Y→∇ YY. f + g \;\coloneqq\; X \overset{\Delta_X}{\to} X \times X \simeq X \oplus X \overset{f \oplus g}{\longrightarrow} Y \oplus Y \simeq Y \sqcup Y \overset{\nabla_Y}{\to} Y \,.With respect to this operation, composition is bilinear. ProofThe associativity and commutativity of ++ follows directly from the corresponding properties of ⊕\oplus. Bilinearity of composition follows from naturality of the diagonal Δ X\Delta_X and codiagonal ∇ X\nabla_X: W ⟶Δ W W×W ⟶≃ W⊕W ↓ e ↓ e×e ↓ e⊕e X →Δ X X×X ≃ X⊕X ⟶f⊕g Y⊕Y ≃ Y⊔Y →∇ X Y ↓ h⊕h ↓ h⊔h ↓ h Z⊕Z ≃ Z⊔Z →∇ Z Z \array{ W &\overset{\Delta_W}{\longrightarrow}& W \times W &\overset{\simeq}{\longrightarrow}& W \oplus W \\ \downarrow^{\mathrlap{e}} && \downarrow^{\mathrlap{e \times e}} && \downarrow^{\mathrlap{e \oplus e}} \\ X &\overset{\Delta_X}{\to}& X \times X &\simeq& X \oplus X &\overset{f \oplus g}{\longrightarrow}& Y \oplus Y &\simeq& Y \sqcup Y &\overset{\nabla_X}{\to}& Y \\ && && && \downarrow^{\mathrlap{h \oplus h}} && \downarrow^{\mathrlap{h \sqcup h}} && \downarrow^{\mathrlap{h}} \\ && && && Z \oplus Z &\simeq& Z \sqcup Z &\overset{\nabla_Z}{\to}& Z } PropositionGiven an additive category according to def. , then the enrichment in commutative monoids which is induced on it via prop. and prop. from its underlying semiadditive category structure coincides with the original enrichment. ProofBy the proof of prop. , the codiagonal on any object in an additive category is the sum of the two projections: ∇ X:X⊕X⟶p 1+p 2X. \nabla_X \;\colon\; X \oplus X \overset{p_1 + p_2}{\longrightarrow} X \,.Therefore (checking on generalized elements, as in the proof of prop. ) for all morphisms f,g:X→Yf,g \colon X \to Y we have commuting squares of the form X ⟶f+g Y Δ X↓ ↑ p 1+p 2 ∇ Y= X⊕X ⟶f⊕g Y⊕Y. \array{ X &\overset{f+g}{\longrightarrow}& Y \\ {}^{\mathllap{\Delta_X}}\downarrow && \uparrow^{\mathrlap{\nabla_Y =}}_{\mathrlap{p_1 + p_2}} \\ X \oplus X &\underset{f \oplus g}{\longrightarrow}& Y\oplus Y } \,. RemarkProp. says that being an additive category is an extra property on a category, not extra structure. We may ask whether a given category is additive or not, without specifying with respect to which abelian group structure on the hom-sets. Related conceptssemiadditive category abelian category additive (∞,1)-category triangulated category ReferencesTextbook accounts: Masaki Kashiwara, Pierre Schapira, Section 8 of: Categories and Sheaves, Grundlehren der Mathematischen Wissenschaften 332, Springer (2006) [doi:10.1007/3-540-27950-4, pdf]Discussion in homological algebra: Charles Weibel, An Introduction to Homological Algebra, Cambridge University Press (1994) [doi:10.1017/CBO9781139644136, pdf]See also: William Lawvere, Introduction to Linear Categories and Applications, course lecture notes (1992) [pdf, pdf]Discussion of model category structures on additive categories is around def. 4.3 of Apostolos Beligiannis, Homotopy theory of modules and Gorenstein rings, Math. Scand. 89 (2001) (pdf)Formalization of additive categories as univalent categories in univalent foundations of mathematics (homotopy type theory): unimath, Additive categories [UniMath.CategoryTheory.Additive]A characterisation of preadditive categories in terms of commutative monoids in cartesian multicategories is given in: Claudio Pisani, Sequential multicategories, Theory and Applications of Categories 29.19 (2014), arXiv:1402.0253Last revised on January 25, 2024 at 09:12:00. See the history of this page for a list of all contributions to it. EditDiscussPrevious revisionChanges from previous revisionHistory (45 revisions) Cite Print Source |
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