Formal Definition
Let X be a locally path connected topological space, and M a module over some ring R. A local coefficient system of R-modules E with fiber M is a locally trivial fibration (i.e. a fiber bundle) with fiber M with an action of the fundamental groupoid of the base X, that is, for each path, a morphism that depends only on the homotopy class with fixed extremities of the path, is the identity on constant paths and such that composition of paths corresponds to compositions of morphisms.
In sheaf theory terms, a constant sheaf has locally constant functions as its sections. Consider instead a sheaf F, such that locally on X it is a constant sheaf. That means that in some neighbourhood of any x in X, it is isomorphic to a constant sheaf. Then F may be used as a system of local coefficients on X.
Read more about this topic: Local System
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