Notational conventions:
- Finite direct products: \(\bigoplus\)
- Cohomological indexing: \(C^i, {{\partial}}^i\)
- Homological indexing: \(C_i, {{\partial}}_i\)
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Right-derived functors \(R^iF\).
- Come from left-exact functors
- Require injective resolutions
- Extend to the right: \(0 \to F(A) \to F(B) \to F(C) \to L_1 F(A) \cdots\)
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Left-derived functors \(L_i F\).
- Come from right-exact functors
- Require projective resolutions
- Extend to the left: \(\cdots L_1F(C) \to F(A) \to F(B) \to F(C) \to 0\)
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Colimits:
- Examples: coproducts, direct limits, cokernels, initial objects, pushouts
- Commute with left adjoints, i.e. \(L(\colim F_i) = \colim LF_i\).
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Examples of limits:
- Products, inverse limits, kernels, terminal objects, pullbacks
- Commute with right adjoints. i.e. \(R(\colim F_i) = \colim RF_i\).
A chain complex \(C\) is acyclic if and only if \(H_*(C) = 0\).
\envlist
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Free \(\implies\) projective \(\implies\) flat \(\implies\) torsionfree (for finitely-generated \(R{\hbox{-}}\)modules)
- Over \(R\) a PID: free \(\iff\) torsionfree
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On limits:
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Limits commute with limits, and colimits commute with colimits.
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Generally, limits do not commute with colimits.
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In \({\mathsf{Set}}\), filtered colimits commute with finite limits.
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In \({\mathsf{Ab}}\), direct colimits commute with finite limits. Inverse limits do not generally commute with finite colimits.
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On adjoints:
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Left adjoints are right-exact with left-derived functors. Right adjoints are left-exact with right-derived functors.
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Left adjoints commute with colimits: \(L( \colim F) = \colim (L\circ F)\) In \({\mathsf{Ab}}\), direct colimits commute with finite limits. Inverse limits do not generally commute with finite colimits.
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Left adjoints are right-exact with left-derived functors. Right adjoints are left-exact with right-derived functors.
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Left adjoints commute with colimits: \(L( \colim F) = \colim (L\circ F)\)
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Hom and Ext
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\(\mathop{\mathrm{Hom}}_R(A, {-})\) is:
- Covariant
- Left-exact
- Is a functor that sends \(f:X\to Y\) to \(f_*: \mathop{\mathrm{Hom}}(A, X) \to \mathop{\mathrm{Hom}}(A, Y)\) given by \(f_*(h) = f\circ h\).
- Has right-derived functors \(\operatorname{Ext} ^i_R(A, B) \coloneqq R^i \mathop{\mathrm{Hom}}_R(A, {-})(B)\) computed using injective resolutions.
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\(\mathop{\mathrm{Hom}}_R({-}, B)\) is:
- Contravariant
- Right-exact
- Is a functor that sends \(f:X\to Y\) to \(f^*: \mathop{\mathrm{Hom}}(Y, B) \to \mathop{\mathrm{Hom}}(X, B)\) given by \(f^*(h) = h\circ f\).
- Has left-derived functors \(\operatorname{Ext} ^i_R(A, B) \coloneqq L_i \mathop{\mathrm{Hom}}_R({-}, B)(A)\) computed using projective resolutions.
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For \(N \in ({R}, {S'}){\hbox{-}}\mathsf{biMod}\) and \(M\in ({R}, {S}){\hbox{-}}\mathsf{biMod}\), \(\mathop{\mathrm{Hom}}_R(M, N) \in ({S}, {S'}){\hbox{-}}\mathsf{biMod}\).
- Mnemonic: the slots of \(\mathop{\mathrm{Hom}}_R\) use up a left \(R{\hbox{-}}\)action. In the first slot, the right \(S{\hbox{-}}\)action on \(M\) becomes a left \(S{\hbox{-}}\)action on Hom. In the second slot, the right \(S'{\hbox{-}}\)action on \(N\) becomes a right \(S'{\hbox{-}}\)action on Hom.
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- \(\operatorname{Ext} ^{>1}(A, B) = 0\) for any \(A\) projective or \(B\) injective.
A maps \(A \xrightarrow{f} B\) in \({\mathsf{R}{\hbox{-}}\mathsf{Mod}}\) is injective if and only if \(f(a) = 0_B \implies a = 0_A\). Monomorphisms are injective maps in \({\mathsf{R}{\hbox{-}}\mathsf{Mod}}\).
Write \(F({-}) \coloneqq\mathop{\mathrm{Hom}}_R(A, {-})\). This is left-exact and thus has right-derived functors \(\operatorname{Ext} ^i_R(A, B) \coloneqq R^iF(B)\). To compute this:
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Take an injective resolution: \begin{align*} 1 \to B \xrightarrow{{\varepsilon}} I^0 \xrightarrow{d^0} I^1 \xrightarrow{d^1} \cdots .\end{align*}
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Remove the augmentation \({\varepsilon}\) and just keep the complex \begin{align*} I^{-}\coloneqq\qty{ 1 \xrightarrow{d^{-1}} I^0 \xrightarrow{d^0} I^1 \xrightarrow{d^1} \cdots } .\end{align*}
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Apply \(F({-})\) to get a new (and usually not exact) complex \begin{align*} F(I)^{-}\coloneqq\qty{ 1 \xrightarrow{{{\partial}}^{-1}} F(I^0) \xrightarrow{{{\partial}}^0} F(I^1) \xrightarrow{{{\partial}}^1} \cdots } ,\end{align*} where \({{\partial}}^i \coloneqq F(d^i)\).
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Take homology, i.e. kernels mod images: \begin{align*} R^iF(B) \coloneqq{ \ker d^i \over \operatorname{im}d^{i-1}} .\end{align*}
Note that \(R^0 F(B) \cong F(B)\) canonically:
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This is defined as \(\ker {{\partial}}^0 / \operatorname{im}{{\partial}}^{-1} = \ker {{\partial}}^0 / 1 = \ker {{\partial}}^0\).
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Use the fact that \(F({-})\) is left exact and apply it to the augmented complex to obtain \begin{align*} 1 \to F(B) \xrightarrow{F({\varepsilon})} F(I^0) \xrightarrow{{{\partial}}^0} F(I^1) \xrightarrow{{{\partial}}^1} \cdots .\end{align*}
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By exactness, there is an isomorphism \(\ker {{\partial}}^0 \cong F(B)\).
\(\phi: \mathop{\mathrm{Hom}}_{{\mathbb{Z}}}({\mathbb{Z}}, {\mathbb{Z}}/n) \xrightarrow{\sim} {\mathbb{Z}}/n\), where \(\phi(g) \coloneqq g(1)\).
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That this is an isomorphism follows from
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Surjectivity: for each \(\ell \in {\mathbb{Z}}/n\) define a map \begin{align*} \psi_y: {\mathbb{Z}}&\to {\mathbb{Z}}/n \\ 1 &\mapsto [\ell]_n .\end{align*}
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Injectivity: if \(g(1) = [0]_n\), then \begin{align*} g(x) = xg(1) = x[0]_n = [0]_n .\end{align*}
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\({\mathbb{Z}}{\hbox{-}}\)module morphism: \begin{align*} \phi(gf) \coloneqq\phi(g\circ f) \coloneqq(g\circ f)(1) = g(f(1)) = f(1)g(1) = \phi(g)\phi(f) ,\end{align*} where we’ve used the fact that \({\mathbb{Z}}/n\) is commutative.
- \(\mathop{\mathrm{Hom}}_{\mathbb{Z}}({\mathbb{Z}}/m, {\mathbb{Z}}) = 0\).
- \(\mathop{\mathrm{Hom}}_{\mathbb{Z}}({\mathbb{Z}}/m, {\mathbb{Z}}/n) = {\mathbb{Z}}/d\).
- \(\mathop{\mathrm{Hom}}_{\mathbb{Z}}({\mathbb{Q}}, {\mathbb{Q}}) = {\mathbb{Q}}\).
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\(\operatorname{Ext} _{\mathbb{Z}}({\mathbb{Z}}/m, G) \cong G/mG\)
- Use \(1 \to {\mathbb{Z}}\xrightarrow{\times m} {\mathbb{Z}}\xrightarrow{} {\mathbb{Z}}/m \to 1\) and apply \(\mathop{\mathrm{Hom}}_{\mathbb{Z}}({-}, {\mathbb{Z}})\).
- \(\operatorname{Ext} _{\mathbb{Z}}({\mathbb{Z}}/m, {\mathbb{Z}}/n) = {\mathbb{Z}}/d\).
TFAE in \({\mathsf{R}{\hbox{-}}\mathsf{Mod}}\):
- A SES \(0\to A\to B \to C\to 0\) is split.
- ?
Tensor and Tor
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\(A\otimes_R {-}\) is:
- Covariant
- Right-exact
- Has left-derived functors \(\operatorname{Ext} ^i_R(A, B) \coloneqq L_i \mathop{\mathrm{Hom}}_R({-}, B)(A)\) computed using projective resolutions.
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\({-}\otimes_R B\) is:
- Covariant
- Right-exact
- Has left-derived functors \(\operatorname{Ext} ^i_R(A, B) \coloneqq L_i \mathop{\mathrm{Hom}}_R({-}, B)(A)\) computed using projective resolutions.
- Tensor is a colimit, and colimits commute with colimits: \((\colim A_i)\otimes_R M = \colim (A_i \otimes_R M)\).
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- \(\operatorname{Tor}_n^R(A, B) = 0\) for either \(A\) or \(B\) flat.
The most useful SES for proofs here: \begin{align*} 0 \to {\mathbb{Z}}\xrightarrow{n} {\mathbb{Z}}\xrightarrow{\pi} {\mathbb{Z}}/n \to 0 .\end{align*}
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- \({\mathbb{Z}}/n \otimes_{\mathbb{Z}}G \cong G/nG\)
- \({\mathbb{Z}}/n \otimes_{\mathbb{Z}}{\mathbb{Z}}/m \cong {\mathbb{Z}}/d\).
- \({\mathbb{Q}}\otimes_{\mathbb{Z}}{\mathbb{Z}}/n \cong 0\).
- \(\operatorname{Tor}^{\mathbb{Z}}_1({\mathbb{Z}}/n, G) \cong \left\{{ h\in H {~\mathrel{\Big\vert}~}nh = e }\right\}\)
- \(\operatorname{Tor}^{\mathbb{Z}}_1({\mathbb{Z}}/n, {\mathbb{Q}}) \cong 0\).
- \(\operatorname{Tor}^{\mathbb{Z}}_1({\mathbb{Z}}/n, {\mathbb{Z}}/m) \cong {\mathbb{Z}}/d\).
Universal Properties
If \(f: G\to K\) and \(H{~\trianglelefteq~}G\) (so that \(G/H\) is defined), then the map \(f\) descends to the quotient if and only if \(H \subseteq \ker(f)\).
The kernel \(\ker f\) of a morphism \(f\) can be characterized as a cartesian square, and the cokernel \(\operatorname{coker}f\) as a cocartesian square:
Adjunctions
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For a fixed \(M\in ({R}, {S}){\hbox{-}}\mathsf{biMod}\), there is an adjunction \begin{align*} \adjunction{ {-}\otimes_R M }{\mathop{\mathrm{Hom}}_S(M, {-})}{ \mathsf{Mod}{\hbox{-}}\mathsf{R} } { \mathsf{Mod}{\hbox{-}}\mathsf{S} } ,\end{align*} so for \(Y \in ({A}, {R}){\hbox{-}}\mathsf{biMod}\) and \(Z \in ({B}, {S}){\hbox{-}}\mathsf{biMod}\), there is a (natural) isomorphism in \(({B}, {A}){\hbox{-}}\mathsf{biMod}\): \begin{align*} \mathop{\mathrm{Hom}}_S(X \otimes_R M, Z) \xrightarrow{\sim} \mathop{\mathrm{Hom}}_R( X, \mathop{\mathrm{Hom}}_S(M, Z) ) .\end{align*}
Let \(F: {\mathsf{R}{\hbox{-}}\mathsf{Mod}} \to {\mathsf{{\mathbb{Z}}}{\hbox{-}}\mathsf{Mod}}\) be the forgetful functor, then there are adjunctions \begin{align*} \adjunction{F}{ \mathop{\mathrm{Hom}}_{\mathbb{Z}}(R, {-})} {{\mathsf{R}{\hbox{-}}\mathsf{Mod}} } {{\mathsf{{\mathbb{Z}}}{\hbox{-}}\mathsf{Mod}} } \\ \\ \adjunction{R\otimes_{\mathbb{Z}}{-}}{F}{ {\mathsf{{\mathbb{Z}}}{\hbox{-}}\mathsf{Mod}} }{ {\mathsf{R}{\hbox{-}}\mathsf{Mod}} } .\end{align*}