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We analyze the Bianchi Identity as an instance of a basic fact of combinatorial groupoid theory, related to the Homotopy Addition Lemma. Here it becomes formulated in terms of 2-forms with values in the gauge group bundle of a groupoid, and leads in particular to the (Chern-Weil) construction of characteristic classes. The method is that of synthetic differential geometry, using "the first neighbourhood of the diagonal" of a manifold as its basic combinatorial structure. We introduce as a tool a new and simple description of wedge (= exterior) products of differential forms in this context.
@article{TAC_1996_2_a6, author = {Anders Kock}, title = {Combinatorics of curvature, and the {Bianchi} identity}, journal = {Theory and applications of categories}, pages = {69--89}, publisher = {mathdoc}, volume = {2}, year = {1996}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_1996_2_a6/} }
Anders Kock. Combinatorics of curvature, and the Bianchi identity. Theory and applications of categories, Tome 2 (1996), pp. 69-89. http://geodesic.mathdoc.fr/item/TAC_1996_2_a6/