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We study spans of cospans in a category C and explain how to horizontally and vertically compose these. When C is a topos and the legs of the spans are monic, these two forms of composition satisfy the interchange law. In this case there is a bicategory of objects, cospans, and `monic-legged' spans of cospans in C. One motivation for this construction is an application to graph rewriting.
@article{TAC_2018_33_a5, author = {Daniel Cicala}, title = {Spans of cospans}, journal = {Theory and applications of categories}, pages = {131--147}, publisher = {mathdoc}, volume = {33}, year = {2018}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2018_33_a5/} }
Daniel Cicala. Spans of cospans. Theory and applications of categories, Tome 33 (2018), pp. 131-147. http://geodesic.mathdoc.fr/item/TAC_2018_33_a5/