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Local system
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<p>In <a href="/facts/Mathematics/pxTouaz4">mathematics</a>, a local system (or a system of local coefficients) on a <a href="/facts/Topological_space/v2ut7CbK">topological space</a> <i>X</i> is a tool from <a href="/facts/Algebraic_topology/2wA8yrjT">algebraic topology</a> which interpolates between <a href="/facts/Homology_theory/VFJmryEW">cohomology</a> with coefficients in a fixed <a href="/facts/Abelian_group/FfWwmx4R">abelian group</a> <i>A</i>, and general <a href="/facts/Sheaf_cohomology/TrcpmD6k">sheaf cohomology</a> in which coefficients vary from point to point. Local coefficient systems were introduced by <a href="/facts/Norman_Steenrod/Hlsfqrye">Norman Steenrod</a> in 1943. </p><p>Local systems are the building blocks of more general tools, such as <a href="/facts/Constructible_sheaf/RkmD0R9H">constructible</a> and <a href="/facts/Perverse_sheaf/HyvX2c0Q">perverse</a> sheaves. </p>