nLab
local system

Idea

A local system – which is short for local system of coefficients for cohomology – is a locally constant sheaf. Cohomology with coefficients in a local system is the corresponding sheaf cohomology.

More generally, we say a local system is a locally constant stack, … and eventually a locally constant ∞-stack.

Under suitable conditions (if we have Galois theory) local systems on $X$ correspond to functors out of the fundamental groupoid of $X$, or more generally to (∞,1)-functors out of the fundamental ∞-groupoid.

Definitions

A notion of cohomology exists intrinsically within any (∞,1)-topos. We discuss local systems first in this generality and then look at special cases, such as local systems as ordinary sheaves.

General

For $H$ an (∞,1)-sheaf (∞,1)-topos, write

$$(\mathrm{LConst}⊣\Gamma ):H\stackrel{\stackrel{\mathrm{LConst}}{\leftarrow }}{\underset{\mathrm{LConst}}{\to }}\mathrm{\infty }\mathrm{Grpd}$$(LConst \dashv \Gamma) : \mathbf{H} \stackrel{\overset{LConst}{\leftarrow}}{\underset{LConst}{\to}} \infty Grpd

be the terminal (∞,1)-geometric morphism, where $\Gamma$ is the global section (∞,1)-functor and $\mathrm{LConst}$ the constant ∞-stack-functor.

Write $𝒮:=\mathrm{core}(\mathrm{Fin}\mathrm{\infty }\mathrm{Grpd})\in$ ∞Grpd for the core ∞-groupoid of the (∞,1)-category of finite $\mathrm{\infty }$-groupoids. (We can drop the finiteness condition by making use of a higher universe.) This is canonically a pointed object $*\to 𝒮$, with points the terminal groupoid.

Definition

For $X\in H$ an object, a local system of locally constant ∞-stack on $X$ is a morphism

$$\tilde{\nabla }:X\to \mathrm{LConst}𝒮$$\tilde \nabla : X \to LConst \mathcal{S}

in $H$ or equivalently the object in the over-(∞,1)-topos

$$(P\to X)\in H/X$$(P \to X) \in \mathbf{H}/X

that is classified by $\tilde{\nabla }$ under the (∞,1)-Grothendieck construction

$$\begin{array}{ccc}P& \to & \mathrm{LConst}𝒵\\ \downarrow & & \downarrow \\ X& \stackrel{\tilde{\nabla }}{\to }& \mathrm{LConst}𝒮\end{array}$$\array{ P &\to& LConst \mathcal{Z} \\ \downarrow && \downarrow \\ X &\stackrel{\tilde \nabla}{\to}& LConst \mathcal{S} }

In other words, local systems are locally constant ∞-stacks or cocycles for cohomology with constant coefficients.

See principal ∞-bundle for discussion of how cocycles $\tilde{\nabla }:X\to \mathrm{LConst}𝒮$ classiy morphisms $P\to X$.

Remark

If $H$ happens to be a locally ∞-connected (∞,1)-topos in that there is the further left adjoint (∞,1)-functor $\Pi$

$$(\Pi ⊣\mathrm{LConst}⊣\Gamma ):H\to \mathrm{\infty }\mathrm{Grpd}$$(\Pi \dashv LConst \dashv \Gamma) : \mathbf{H} \to \infty Grpd

we call $\Pi (X)$ the fundamental ∞-groupoid in a locally ∞-connected (∞,1)-topos. In this case, by the adjunction hom-equivalence we have

$$H(X,\mathrm{LConst}𝒮)\simeq \mathrm{Func}(\Pi (X),𝒮).$$\mathbf{H}(X, LConst \mathcal{S}) \simeq Func(\Pi(X), \mathcal{S}) \,.

This means that local systems are naturally identified with representations ($\mathrm{\infty }$-permutation representations, as it were) of the fundamental $\mathrm{\infty }$-groupoid.

The (∞,1)-sheaf (∞,1)-topos over a locally contractible space is locally $\mathrm{\infty }$-connected, and many authors identify local systems on such a topological space with representations of its fundamental groupoid.

Definition

Given a local system $\tilde{\nabla }:X\to \mathrm{LConst}𝒮$, the cohomology of $X$ with this local system of coefficients is the intrinsic cohomology of the over-(∞,1)-topos $H/X$:

$$H(X,\tilde{\nabla }):=H_{/X}(X,P_{\tilde{\nabla }}),$$H(X,\tilde \nabla) := \mathbf{H}_{/X}(X, P_{\tilde \nabla}) \,,

where $P_{\tilde{\nabla }}$ is the homotopy fiber of $\tilde{\nabla }$.

Remark

Unwinding the definitions and using the universality of the (∞,1)-pullback, one sees that a cocycle $c\in H(X,\tilde{\nabla })$ is a diagram

$$\begin{array}{ccccc}X& & \stackrel{c}{\to }& & *\\ & \searrow & ⇙& \swarrow \\ & & \mathrm{LConst}𝒮\end{array}$$\array{ X &&\stackrel{c}{\to}&& * \\ & \searrow &\swArrow& \swarrow \\ && LConst \mathcal{S} }

in $H$. This is precisely a section of the locally constant ∞-stack $\tilde{\nabla }$.

Sheaf-theoretic case

Local systems can also be considered in abelian contexts. One finds the following version of a local system

Definition

A linear local system is a locally constant sheaf on a topological space $X$ (or manifold, analytic manifold, or algebraic variety) whose stalk is a finite-dimensional vector space.

Regarded as a sheaf $F$ with values in abelian groups, such a linear local system serves as the coefficient for abelian sheaf cohomology. As discussed there, this is in degree $n$ nothing but the intrinsic cohomology of the $\mathrm{\infty }$-topos with coefficients in the Eilenberg-MacLane object $B^{n}F$.

Lemma

On a connected topological space this is the same as a sheaf of sections of a finite-dimensional vector bundle equipped with flat connection on a bundle; and it also corresponds to the representations of the fundamental group ${\pi }_1(X,x_0)$ in the typical stalk. On an analytic manifold or a variety, there is an equivalence between the category of non-singular coherent $D_X$-modules and local systems on $X$.

Related concepts

• simplicial local system: within Sullivan’s (1977) theory of Infinitesimal computations in topology, he refers to ‘local systems’ several times. This seems to be simplicial in nature. This entry explores some of the uses of that notion based on Halperin’s lecture notes on minimal models

  • D. Sullivan, Infinitesimal computations in topology (pdf)

References

An early version of the definition of local system appears in

• Norman Steenrod: Homology with local coefficients, Annals 44 (1943) pp. 610 - 627,

This is before the formal notion of sheaf was published by Jean Leray. (Wikipedia’s entry on Sheaf theory is interesting for its historical perspective on this.)

A definition appears as an exercise in

• Edwin Spanier, 1966, Algebraic Topology , McGraw Hill. (republished by Springer, 1982).

on page 58 :

A local system on a space $X$ is a covariant functor from the fundamental groupoid of $X$ to some category.

A blog exposition of some aspects of linear local system is developed here:

• David Speyer, Three ways of looking at a local system

  • Introduction and connection to cohomology theories

  • the path groupoid approach

  • the infinitesimal perspective

  • the infinitesimal site

A clear-sighted description of locally constant $(n-1)$-stacks / $n$-local systems as sections of constant $n$-stacks is in

• Pietro Polesello, Ingo Waschkies, Higher monodromy, Homology, Homotopy and Applications, Vol. 7(2005), No. 1, pp. 109-150; arXiv:0407507

for locally constant stacks on topological spaces. The above formulation is pretty much the evident generalization of this to general (∞,1)-toposes.