In mathematics, the first Blakers–Massey theorem, named after Albert Blakers and William S. Massey,[1][2][3] gave vanishing conditions for certain triad homotopy groups of spaces.

Description of the result

This connectivity result may be expressed more precisely, as follows. Suppose X is a topological space which is the pushout of the diagram

,

where f is an m-connected map and g is n-connected. Then the map of pairs

induces an isomorphism in relative homotopy groups in degrees and a surjection in the next degree.

However the third paper of Blakers and Massey in this area[4] determines the critical, i.e., first non-zero, triad homotopy group as a tensor product, under a number of assumptions, including some simple connectivity. This condition and some dimension conditions were relaxed in work of Ronald Brown and Jean-Louis Loday.[5] The algebraic result implies the connectivity result, since a tensor product is zero if one of the factors is zero. In the non simply connected case, one has to use the nonabelian tensor product of Brown and Loday.[5]

The triad connectivity result can be expressed in a number of other ways, for example, it says that the pushout square above behaves like a homotopy pullback up to dimension .

Generalization to higher toposes

The generalization of the connectivity part of the theorem from traditional homotopy theory to any other infinity-topos with an infinity-site of definition was given by Charles Rezk in 2010.[6]

Fully formal proof

In 2013 a fairly short, fully formal proof using homotopy type theory as a mathematical foundation and an Agda variant as a proof assistant was announced by Peter LeFanu Lumsdaine;[7] this became Theorem 8.10.2 of Homotopy Type Theory – Univalent Foundations of Mathematics.[8] This induces an internal proof for any infinity-topos (i.e. without reference to a site of definition); in particular, it gives a new proof of the original result.

References

  1. ↑ Blakers, Albert L.; Massey, William S. (1949). "The homotopy groups of a triad". Proceedings of the National Academy of Sciences of the United States of America. 35 (6): 322–328. Bibcode:1949PNAS...35..322B. doi:10.1073/pnas.35.6.322. MR 0030757. PMC 1063027. PMID 16588898.
  2. ↑ Blakers, Albert L.; Massey, William S. (1951), "The homotopy groups of a triad. I", Annals of Mathematics, (2), 53 (1): 161–204, doi:10.2307/1969346, JSTOR 1969346, MR 0038654
  3. ↑ Hatcher, Allen, Algebraic Topology, Theorem 4.23
  4. ↑ Blakers, Albert L.; Massey, William S. (1953). "The homotopy groups of a triad. III". Annals of Mathematics. (2). 58 (3): 409–417. doi:10.2307/1969744. JSTOR 1969744. MR 0058971.
  5. 1 2 Brown, Ronald; Loday, Jean-Louis (1987). "Homotopical excision, and Hurewicz theorems, for n-cubes of spaces". Proceedings of the London Mathematical Society. (3). 54 (1): 176–192. doi:10.1112/plms/s3-54.1.176. MR 0872255.
  6. ↑ Rezk, Charles (2010). "Toposes and homotopy toposes" (PDF). Prop. 8.16.
  7. ↑ "The Blakers-Massey theorem in homotopy type theory (talk at Conference on Type Theory, Homotopy Theory and Univalent Foundations)". 2013.
  8. ↑ The Univalent Foundations Program (2013). Homotopy type theory: Univalent foundations of mathematics. Institute for Advanced Study.
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