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Ivancevic_Applied-Diff-Geom

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98 <strong>Applied</strong> <strong>Diff</strong>erential <strong>Geom</strong>etry: A Modern Introduction(2)G × G × Gµ × 1 ✲ G × G1 × µ❄G × Gµµ❄✲ G(3)(associativity)G(ν, 1)✲G × G(1, ν)✲ G❅e❅ µ e❅❅❘❄ ✒G(inverse).Here e : G → G is the constant map e(g) = e for all g ∈ G. (e, 1) meansthe map such that (e, 1)(g) = (e, g), etc. A group G is called commutativeor Abelian group if in addition the following diagram commutesG × GT❅µ ❅❅ ❅❘G✲ G × Gµ ✠where T : G × G → G × G is the switch map T (g 1 , g 2 ) = (g 1 , g 2 ), for all(g 1 , g 2 ) ∈ G × G.A group G acts (on the left) on a set A if there is a function α : G×A →A such that the following diagrams commute [Switzer (1975)]:(1)A(e, 1)✲G × A❅❅❅❅❘A1α❄

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