Paths and cylinders
(∞,1)-category of (∞,1)-sheaves
Extra stuff, structure and property
locally n-connected (n,1)-topos
locally ∞-connected (∞,1)-topos, ∞-connected (∞,1)-topos
structures in a cohesive (∞,1)-topos
In abelian categories one talks of chain complexes; and in that context a composable pair is null iff factors through the cokernel :
and so forth. In a strict context, the factorization is unique.
In a pointed (∞,1)-category with (∞,1)-colimits of small 1-truncated diagrams, one may still consider factorizations through cofibers: but now there is a choice to make, roughly parametrized by an action of . This leads to interesting structure, describing (with upper bounds!) how trivially a particular sequence of arrows may compose.
To begin, consider a sequence of maps . If the composites and are nulhomotopic, then one has a diagram
any choice of homotopies in the two squares gives a map .
Define and to be the cofibers of and , respectively. A choice of homotopy corresponds to a choice of factorization , which gives a diagram of pushout squares
It is to be noted that the map and possibly the object depend on the choice of factor , but that does not, in any meaningful sense, so depend: this is just the structure map of the cofiber of . Note that the cofiber of is thus equivalent to that of ; but again the role of choices must be studied.
A sequence of maps will be called a bracket sequence (a novel phrase for the purposes of this entry) in either of two cases:
- and the composites and are nulhomotopic; OR
- , and (using the preceding notations), there are choices of factor and such that the induced sequence is a bracket sequence.
In all cases, a bracket sequence leads to a three-map sequence
in which consecutive maps compose trivially, and so there are induced choices of maps
The collection of all such maps, taking all compatible variations, is the Toda Bracket of the bracket sequence.
Among the bracket sequences, a particular family arises which here will be called null-bracket (again, a novel phrase). A sequence will be called null-bracket if
- and is trivial, OR
- , and there is a choice of factorization such that the sequence is null-bracket.
If the Toda bracket for a bracket sequence includes the trivial map then the sequence is null-bracket.
By definition, if a sequence is a bracket sequence AND NOT a null-bracket sequence, it follows that all the relevant maps are nontrivial. Things like these Toda brackets have been studied by many (FIXME: referrences later) and especially the length-three brackets used by H. Toda to describe most of for or so.
In (Cohen, 1968) is given a criterion for stable maps of spheres to inhabit non-null Toda brackets; this turns out to be most of , and furthermore the maps in the bracket sequences can be chosen from a very small set (_FIXME_: be more precise! degree maps , Hopf maps , and … )
- Joel Cohen, The decomposition of stable homotopy, Annals of Mathematics (2) 87 (2): 305–320 (1968)
- Hans-Joachim Baues, On the cohomology of categories, universal Toda brackets and homotopy pairs, K-Theory 11:3, April 1997, pp. 259-285 (27) springer
- Boryana Dimitrova, Universal Toda brackets of commutative ring spectra, poster, Bonn 2010, pdf
- C. Roitzheim?, S. Whitehouse, Uniqueness of -structures and Hochschild cohomology, arxiv/0909.3222
- Steffen Sagave, Universal Toda brackets of ring spectra, Trans. Amer. Math. Soc., 360(5):2767-2808, 2008, math.KT/0611808