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  2. Retraction (topology) - Wikipedia

    en.wikipedia.org/wiki/Retraction_(topology)

    A space is an absolute neighborhood retract for the class , written ⁡ (), if is in and whenever is a closed subset of a space in , is a neighborhood retract of . Various classes C {\displaystyle {\mathcal {C}}} such as normal spaces have been considered in this definition, but the class M {\displaystyle {\mathcal {M}}} of metrizable spaces ...

  3. Borsuk–Ulam theorem - Wikipedia

    en.wikipedia.org/wiki/Borsuk–Ulam_theorem

    The Borsuk–Ulam theorem has several equivalent statements in terms of odd functions. Recall that S n {\displaystyle S^{n}} is the n -sphere and B n {\displaystyle B^{n}} is the n -ball : If g : S n → R n {\displaystyle g:S^{n}\to \mathbb {R} ^{n}} is a continuous odd function, then there exists an x ∈ S n {\displaystyle x\in S^{n}} such ...

  4. Section (category theory) - Wikipedia

    en.wikipedia.org/wiki/Section_(category_theory)

    Similarly, the natural monomorphism Z/2Z → Z/4Z doesn't split even though there is a non-trivial morphism Z/4Z → Z/2Z. The categorical concept of a section is important in homological algebra , and is also closely related to the notion of a section of a fiber bundle in topology : in the latter case, a section of a fiber bundle is a section ...

  5. Karol Borsuk - Wikipedia

    en.wikipedia.org/wiki/Karol_Borsuk

    Karol Borsuk (8 May 1905 – 24 January 1982) was a Polish mathematician. His main area of interest was topology . He made significant contributions to shape theory , a term which he coined.

  6. Borsuk's conjecture - Wikipedia

    en.wikipedia.org/wiki/Borsuk's_conjecture

    [7] For all n for fields of revolution — shown by Boris Dekster (1995). [8] The problem was finally solved in 1993 by Jeff Kahn and Gil Kalai, who showed that the general answer to Borsuk's question is no. [9] They claim that their construction shows that n + 1 pieces do not suffice for n = 1325 and for each n > 2014.

  7. Bing–Borsuk conjecture - Wikipedia

    en.wikipedia.org/wiki/Bing–Borsuk_conjecture

    In mathematics, the Bing–Borsuk conjecture states that every -dimensional homogeneous absolute neighborhood retract space is a topological manifold. The conjecture has been proved for dimensions 1 and 2, and it is known that the 3-dimensional version of the conjecture implies the Poincaré conjecture .

  8. Shape theory (mathematics) - Wikipedia

    en.wikipedia.org/wiki/Shape_theory_(mathematics)

    Shape theory is a branch of topology that provides a more global view of the topological spaces than homotopy theory. The two coincide on compacta dominated homotopically by finite polyhedra . Shape theory associates with the Čech homology theory while homotopy theory associates with the singular homology theory.

  9. Retract (group theory) - Wikipedia

    en.wikipedia.org/wiki/Retract_(group_theory)

    The following is known about retracts: A subgroup is a retract if and only if it has a normal complement. [4] The normal complement, specifically, is the kernel of the retraction. Every direct factor is a retract. [1] Conversely, any retract which is a normal subgroup is a direct factor. [5] Every retract has the congruence extension property.

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