Local coefficient system

In mathematics, a local system (or a system of local coefficients) on a topological space X is a tool from algebraic topology which interpolates between cohomology with coefficients in a fixed abelian group A, and general sheaf cohomology in which coefficients vary from point to point. Local coefficient systems were introduced by Norman Steenrod in 1943.^[1]

Local systems are the building blocks of more general tools, such as constructible and perverse sheaves.

Let X be a topological space. A local system (of abelian groups/modules...) on X is a locally constant sheaf (of abelian groups/of modules...) on X. In other words, a sheaf ${\displaystyle {\mathcal {L}$ is a local system if every point has an open neighborhood ${\displaystyle U}$ such that the restricted sheaf ${\displaystyle {\mathcal {L}|_{U}$ is isomorphic to the sheafification of some constant presheaf. ^{[clarification needed]}

If X is path-connected,^{[clarification needed]} a local system ${\displaystyle {\mathcal {L}$ of abelian groups has the same stalk ${\displaystyle L}$ at every point. There is a bijective correspondence between local systems on X and group homomorphisms

${\displaystyle \rho :\pi _{1}(X,x)\to {\text{Aut}(L)}$

and similarly for local systems of modules. The map ${\displaystyle \pi _{1}(X,x)\to {\text{Aut}(L)}$ giving the local system ${\displaystyle {\mathcal {L}$ is called the monodromy representation of ${\displaystyle {\mathcal {L}$.

This shows that (for X path-connected) a local system is precisely a sheaf whose pullback to the universal cover of X is a constant sheaf.

This correspondence can be upgraded to an equivalence of categories between the category of local systems of abelian groups on X and the category of abelian groups endowed with an action of ${\displaystyle \pi _{1}(X,x)}$ (equivalently, ${\displaystyle \mathbb {Z} [\pi _{1}(X,x)]}$-modules).^[2]

A stronger nonequivalent definition that works for non-connected X is the following: a local system is a covariant functor

${\displaystyle {\mathcal {L}\colon \Pi _{1}(X)\to {\textbf {Mod}(R)}$

from the fundamental groupoid of ${\displaystyle X}$ to the category of modules over a commutative ring ${\displaystyle R}$, where typically ${\displaystyle R=\mathbb {Q} ,\mathbb {R} ,\mathbb {C} }$. This is equivalently the data of an assignment to every point ${\displaystyle x\in X}$ a module ${\displaystyle M}$ along with a group representation ${\displaystyle \rho _{x}:\pi _{1}(X,x)\to {\text{Aut}_{R}(M)}$ such that the various ${\displaystyle \rho _{x}$ are compatible with change of basepoint ${\displaystyle x\to y}$ and the induced map ${\displaystyle \pi _{1}(X,x)\to \pi _{1}(X,y)}$ on fundamental groups.

• Constant sheaves such as ${\displaystyle {\underline {\mathbb {Q} }_{X}$. This is a useful tool for computing cohomology since in good situations, there is an isomorphism between sheaf cohomology and singular cohomology:

$${\displaystyle H^{k}(X,{\underline {\mathbb {Q} }_{X})\cong H_{\text{sing}^{k}(X,\mathbb {Q} )}$$

• Let ${\displaystyle X=\mathbb {R} ^{2}\setminus \{(0,0)\}$. Since ${\displaystyle \pi _{1}(\mathbb {R} ^{2}\setminus \{(0,0)\})=\mathbb {Z} }$, there is an ${\displaystyle S^{1}$ family of local systems on X corresponding to the maps ${\displaystyle n\mapsto e^{in\theta }$:

$${\displaystyle \rho _{\theta }:\pi _{1}(X;x_{0})\cong \mathbb {Z} \to {\text{Aut}_{\mathbb {C} }(\mathbb {C} )}$$

• Horizontal sections of vector bundles with a flat connection. If ${\displaystyle E\to X}$ is a vector bundle with flat connection ${\displaystyle \nabla }$, then there is a local system given by $${\displaystyle E_{U}^{\nabla }=\left\{\text{sections }s\in \Gamma (U,E){\text{ which are horizontal: }\nabla s=0\right\}$$ For instance, take ${\displaystyle X=\mathbb {C} \setminus 0}$ and ${\displaystyle E=X\times \mathbb {C} ^{n}$, the trivial bundle. Sections of E are n-tuples of functions on X, so ${\displaystyle \nabla _{0}(f_{1},\dots ,f_{n})=(df_{1},\dots ,df_{n})}$ defines a flat connection on E, as does ${\displaystyle \nabla (f_{1},\dots ,f_{n})=(df_{1},\dots ,df_{n})-\Theta (x)(f_{1},\dots ,f_{n})^{t}$ for any matrix of one-forms ${\displaystyle \Theta }$ on X. The horizontal sections are then
  $${\displaystyle E_{U}^{\nabla }=\left\{(f_{1},\dots ,f_{n})\in E_{U}:(df_{1},\dots ,df_{n})=\Theta (f_{1},\dots ,f_{n})^{t}\right\}$$ i.e., the solutions to the linear differential equation ${\displaystyle df_{i}=\sum \Theta _{ij}f_{j}$.

  If ${\displaystyle \Theta }$ extends to a one-form on ${\displaystyle \mathbb {C} }$ the above will also define a local system on ${\displaystyle \mathbb {C} }$, so will be trivial since ${\displaystyle \pi _{1}(\mathbb {C} )=0}$. So to give an interesting example, choose one with a pole at 0:

  $${\displaystyle \Theta ={\begin{pmatrix}0&dx/x\\dx&e^{x}dx\end{pmatrix}$$ in which case for ${\displaystyle \nabla =d+\Theta }$, $${\displaystyle E_{U}^{\nabla }=\left\{f_{1},f_{2}:U\to \mathbb {C} \ \ {\text{ with }f'_{1}=f_{2}/x\ \ f_{2}'=f_{1}+e^{x}f_{2}\right\}$$

• An n-sheeted covering map ${\displaystyle X\to Y}$ is a local system with fibers given by the set ${\displaystyle \{1,\dots ,n\}$. Similarly, a fibre bundle with discrete fibre is a local system, because each path lifts uniquely to a given lift of its basepoint. (The definition adjusts to include set-valued local systems in the obvious way).

• A local system of k-vector spaces on X is equivalent to a k-linear representation of ${\displaystyle \pi _{1}(X,x)}$.

• If X is a variety, local systems are the same thing as D-modules which are additionally coherent O_X-modules (see O modules).

• If the connection is not flat (i.e. its curvature is nonzero), then parallel transport of a fibre F_x over x around a contractible loop based at x_0 may give a nontrivial automorphism of F_x, so locally constant sheaves can not necessarily be defined for non-flat connections.

• The Gauss–Manin connection is a prominent example of a connection whose horizontal sections are studied in relation to variation of Hodge structures.

There are several ways to define the cohomology of a local system, called cohomology with local coefficients, which become equivalent under mild assumptions on X.

• Given a locally constant sheaf ${\displaystyle {\mathcal {L}$ of abelian groups on X, we have the sheaf cohomology groups ${\displaystyle H^{j}(X,{\mathcal {L})}$ with coefficients in ${\displaystyle {\mathcal {L}$.

• Given a locally constant sheaf ${\displaystyle {\mathcal {L}$ of abelian groups on X, let ${\displaystyle C^{n}(X;{\mathcal {L})}$ be the group of all functions f which map each singular n-simplex ${\displaystyle \sigma \colon \Delta ^{n}\to X}$ to a global section ${\displaystyle f(\sigma )}$ of the inverse-image sheaf ${\displaystyle \sigma ^{-1}{\mathcal {L}$. These groups can be made into a cochain complex with differentials constructed as in usual singular cohomology. Define ${\displaystyle H_{\mathrm {sing} }^{j}(X;{\mathcal {L})}$ to be the cohomology of this complex.

• The group ${\displaystyle C_{n}({\widetilde {X})}$ of singular n-chains on the universal cover of X has an action of ${\displaystyle \pi _{1}(X,x)}$ by deck transformations. Explicitly, a deck transformation ${\displaystyle \gamma \colon {\widetilde {X}\to {\widetilde {X}$ takes a singular n-simplex ${\displaystyle \sigma \colon \Delta ^{n}\to {\widetilde {X}$ to ${\displaystyle \gamma \circ \sigma }$. Then, given an abelian group L equipped with an action of ${\displaystyle \pi _{1}(X,x)}$, one can form a cochain complex from the groups ${\displaystyle \operatorname {Hom} _{\pi _{1}(X,x)}(C_{n}({\widetilde {X}),L)}$ of ${\displaystyle \pi _{1}(X,x)}$-equivariant homomorphisms as above. Define ${\displaystyle H_{\mathrm {sing} }^{j}(X;L)}$ to be the cohomology of this complex.

If X is paracompact and locally contractible, then ${\displaystyle H^{j}(X,{\mathcal {L})\cong H_{\mathrm {sing} }^{j}(X;{\mathcal {L})}$.^[3] If ${\displaystyle {\mathcal {L}$ is the local system corresponding to L, then there is an identification ${\displaystyle C^{n}(X;{\mathcal {L})\cong \operatorname {Hom} _{\pi _{1}(X,x)}(C_{n}({\widetilde {X}),L)}$ compatible with the differentials,^[4] so ${\displaystyle H_{\mathrm {sing} }^{j}(X;{\mathcal {L})\cong H_{\mathrm {sing} }^{j}(X;L)}$.

Local systems have a mild generalization to constructible sheaves -- a constructible sheaf on a locally path connected topological space ${\displaystyle X}$ is a sheaf ${\displaystyle {\mathcal {L}$ such that there exists a stratification of

${\displaystyle X=\coprod X_{\lambda }$

where ${\displaystyle {\mathcal {L}|_{X_{\lambda }$ is a local system. These are typically found by taking the cohomology of the derived pushforward for some continuous map ${\displaystyle f:X\to Y}$. For example, if we look at the complex points of the morphism

${\displaystyle f:X={\text{Proj}\left({\frac {\mathbb {C} [s,t][x,y,z]}{(st\cdot h(x,y,z))}\right)\to {\text{Spec}(\mathbb {C} [s,t])}$

then the fibers over

${\displaystyle \mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}$

are the plane curve given by ${\displaystyle h}$, but the fibers over ${\displaystyle \mathbb {V} =\mathbb {V} (st)}$ are ${\displaystyle \mathbb {P} ^{2}$. If we take the derived pushforward ${\displaystyle \mathbf {R} f_{!}({\underline {\mathbb {Q} }_{X})}$ then we get a constructible sheaf. Over ${\displaystyle \mathbb {V} }$ we have the local systems

${\displaystyle {\begin{aligned}\mathbf {R} ^{0}f_{!}({\underline {\mathbb {Q} }_{X})|_{\mathbb {V} (st)}&={\underline {\mathbb {Q} }_{\mathbb {V} (st)}\\\mathbf {R} ^{2}f_{!}({\underline {\mathbb {Q} }_{X})|_{\mathbb {V} (st)}&={\underline {\mathbb {Q} }_{\mathbb {V} (st)}\\\mathbf {R} ^{4}f_{!}({\underline {\mathbb {Q} }_{X})|_{\mathbb {V} (st)}&={\underline {\mathbb {Q} }_{\mathbb {V} (st)}\\\mathbf {R} ^{k}f_{!}({\underline {\mathbb {Q} }_{X})|_{\mathbb {V} (st)}&={\underline {0}_{\mathbb {V} (st)}{\text{ otherwise}\end{aligned}$

while over ${\displaystyle \mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}$ we have the local systems

${\displaystyle {\begin{aligned}\mathbf {R} ^{0}f_{!}({\underline {\mathbb {Q} }_{X})|_{\mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}&={\underline {\mathbb {Q} }_{\mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}\\\mathbf {R} ^{1}f_{!}({\underline {\mathbb {Q} }_{X})|_{\mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}&={\underline {\mathbb {Q} }_{\mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}^{\oplus 2g}\\\mathbf {R} ^{2}f_{!}({\underline {\mathbb {Q} }_{X})|_{\mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}&={\underline {\mathbb {Q} }_{\mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}\\\mathbf {R} ^{k}f_{!}({\underline {\mathbb {Q} }_{X})|_{\mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}&={\underline {0}_{\mathbb {A} _{s,t}^{2}-\mathbb {V} (st)}{\text{ otherwise}\end{aligned}$

where ${\displaystyle g}$ is the genus of the plane curve (which is ${\displaystyle g=(\deg(f)-1)(\deg(f)-2)/2}$).

The cohomology with local coefficients in the module corresponding to the orientation covering can be used to formulate Poincaré duality for non-orientable manifolds: see Twisted Poincaré duality.

• Leray spectral sequence
• Gauss–Manin connection
• D-module
• Intersection homology
• Perverse sheaf

• "What local system really is". Stack Exchange.
• Schnell, Christian. "Computing Cohomology of Local Systems" (PDF). Discusses computing the cohomology with coefficients in a local system by using the twisted de Rham complex.
• Williamson, Geordie. "An illustrated guide to perverse sheaves" (PDF).
• MacPherson, Robert (December 15, 1990). "Intersection homology and perverse sheaves" (PDF).
• El Zein, Fouad; Snoussi, Jawad. "Local systems and constructible sheaves" (PDF).