In category theory, a branch of mathematics, for every object in every category where the product exists, there exists the diagonal morphism[1][2][3][4][5][6]

satisfying

for

where is the canonical projection morphism to the -th component. The existence of this morphism is a consequence of the universal property that characterizes the product (up to isomorphism). The restriction to binary products here is for ease of notation; diagonal morphisms exist similarly for arbitrary products. The image of a diagonal morphism in the category of sets, as a subset of the Cartesian product, is a relation on the domain, namely equality.

For concrete categories, the diagonal morphism can be simply described by its action on elements of the object . Namely, , the ordered pair formed from . The reason for the name is that the image of such a diagonal morphism is diagonal (whenever it makes sense), for example the image of the diagonal morphism on the real line is given by the line that is the graph of the equation . The diagonal morphism into the infinite product may provide an injection into the space of sequences valued in ; each element maps to the constant sequence at that element. However, most notions of sequence spaces have convergence restrictions that the image of the diagonal map will fail to satisfy.

The dual notion of a diagonal morphism is a co-diagonal morphism. For every object in a category where the coproducts exists, the co-diagonal[3][2][7][5][6] is the canonical morphism

satisfying

for

where is the injection morphism to the -th component.

Let be a morphism in a category with the pushout is an epimorphism if and only if the codiagonal is an isomorphism.[8]

See also

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References

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Bibliography

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  • Awodey, s. (1996). "Structure in Mathematics and Logic: A Categorical Perspective". Philosophia Mathematica. 4 (3): 209–237. doi:10.1093/philmat/4.3.209.
  • Baez, John C. (2004). "Quantum Quandaries: A Category-Theoretic Perspective". The Structural Foundations of Quantum Gravity. pp. 240–265. arXiv:quant-ph/0404040. Bibcode:2004quant.ph..4040B. doi:10.1093/acprof:oso/9780199269693.003.0008. ISBN 978-0-19-926969-3.
  • Carter, J. Scott; Crans, Alissa; Elhamdadi, Mohamed; Saito, Masahico (2008). "Cohomology of Categorical Self-Distributivity" (PDF). Journal of Homotopy and Related Structures. 3 (1): 13–63. arXiv:math/0607417. Bibcode:2006math......7417C.
  • Faith, Carl (1973). "Product and Coproduct". Algebra. pp. 83–109. doi:10.1007/978-3-642-80634-6_4. ISBN 978-3-642-80636-0.
  • Kashiwara, Msakia; Schapira, Pierre (2006). "Limits". Categories and Sheaves. Grundlehren der mathematischen Wissenschaften. Vol. 332. pp. 35–69. doi:10.1007/3-540-27950-4_3. ISBN 978-3-540-27949-5.
  • Mitchell, Barry (1965). Theory of Categories. Academic Press. ISBN 978-0-12-499250-4.
  • Muro, Fernando (2016). "Homotopy units in A-infinity algebras". Trans. Amer. Math. Soc. 368: 2145–2184. arXiv:1111.2723. doi:10.1090/tran/6545.
  • Masakatsu, Uzawa (1972). "Some categorical properties of complex spaces Part II" (PDF). Bulletin of the Faculty of Education, Chiba University. 21: 83–93. ISSN 0577-6856.
  • Popescu, Nicolae; Popescu, Liliana (1979). "Categories and functors". Theory of categories. pp. 1–148. doi:10.1007/978-94-009-9550-5_1. ISBN 978-94-009-9552-9.
  • Pupier, R. (1964). "Petit guide des catégories". Publications du Département de Mathématiques (Lyon) (in French). 1 (1): 1–18.
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