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
The Postnikov system is the infinitary factorization system of (n-epi, n-mono)-factorizations through n-images in an (∞,1)-topos.
The basic (and historically first) example is the factorization in ∞Grpd/Top of morphisms to the point: here the Postnikov system assigns to a homotopy type a tower
where each is a homotopy (k-2)-type/(n-2)-truncated object and such that each morphism induces an isomorphism on homotopy groups in degree . This may be thought of as decomposing into its “layers” as seen by the degree of homotopy groups. The characteristic classes which give the ∞-group extension in each layer are called the Postnikov invariants or k-invariants of .
More generally, in any (∞,1)-topos and for any morphism the corresponding Postnikov system is a tower
of n-images of which interpolates between and along : the object is a homotopy type that looks like in low degrees (below ), looks like in high degrees (above degree ) and looks like the ordinary (1-topos theoretic) image of on homotopy groups in degree itself.
In full generality, the Postnikov system is the infinitary factorization system of (n-epi, n-mono)-factorizations through n-images in an (∞,1)-topos.
The following spells this out explicitly for default homotopy theory, hence in ∞Grpd and in terms of the presentation by the model structure on topological spaces and the model structure on simplicial sets:
With a little bit of care this induces corresponding presentations of Postnikov systems in general (∞,1)-topos by prolonging to suitable model structures on simplicial presheaves:
For more discussion of the general abstract situation see at
For topological spaces
Historically, Postnikov systems were first described on the model of homotopy types constituted by topological spaces.
A Postnikov system or Postnikov tower or Moore-Postnikov tower/system of topological spaces, or rather of their homotopy types, is a sequence of path connected, pointed topological spaces , , such that for , together with a sequence of modules of the fundamental group of and fibrations classified up to homotopy type by a specified cohomology class .
For simplicial sets
We discuss the realization of Postnikov systems on simplicial sets/Kan complexes, first in the
and then more generally for the
Let be a simplicial set. A Postnikov tower for is
with maps such that all diagrams
such that for all vertices we have for the homotopy groups
This appears for instance as (GoerssJardine, def VI 3.1).
Let be a homomorphism of simplicial sets. A (relative) Postnikov tower for is a tower
that factors such that for all
induces an epimorphism on homotopy groups in degree ;
induces an isomorphism on homotopy groups in degree
induces a monomorphism on homotopy groups in degree ;
induces an isomorphism on homotopy groups in degree .
This appears for instance as (Goerss-Jardine, def. VI 2.9).
For simplicial presheaves
We discuss explicit constructions/presentations of Postnikov systems.
For simplicial sets
There are three main functorial models for the Postnikov tower of a simplicial set:
If is regarded as an ∞-groupoid modeled as a Kan complex, then the coskeleton sequence
exhibits a Postnikov tower for .
This is observed for instance in (DwyerKan). Also see coskeleton for more details.
Identification relative to skeleta
The following construction quotients out the relations encoded by the cells that are thrown in in the above construction, such as to make the maps in the Postnikov tower into Kan fibrations.
We first discuss the absolute tower and then the relative version.
Absolute Postnikov tower
Let be a Kan complex. Define for each an equivalence relation on the simplices of as follows: two -simplices
are equivalent if their restriction to the -skeleton coincides
for the quotient simplicial set.
There are evident morphisms
This is a Postnikov tower, def. 1, and all morphisms are Kan fibrations.
Moreover the canonical morphism
is an isomorphism, exhibiting as the limit (“inverse limit”) over this tower diagram.
This appears for instance as (GoerssJardine, theorem Vi 3.5).
Relative Postnikov tower
We discuss a model for the relative Postnikov tower, def. 2.
For a Kan fibration between Kan complexes, define for each and each an equivalence relation on -simplices
such that if
the -skeleta are equal;
the images are equal.
be the simplicial set of equivalence classes under this equivalence relation.
This canonically comes with morphisms for .
For instance (Goerss-Jardine, def. VI 2.9).
This construction gives indeed a relative Postnikov tower for .
For instance (Goerss-Jardine, theorem VI 2.11).
Homotopy classes relative to skeleta
For a Kan complex and , let be the simplicial set defined as the quotient
where two -cells are identified if there is a simplicial homotopy between them that fixes their -skeleton.
This is due to John Duskin. See for instance (Beke, pages 302-305).
For strict -groupoids
Consider a morphism in sSet/∞Grpd that is in the image of , hence given by a morphism of strict ω-groupoids.
Then for the n-image factorization/-Postnikov stage of is given by the strict ω-groupoid with
equipped with the evident composition operations induced from those of and , and equipped with the canonical morphisms of strict -groupoids
(the left one being the identity in degree , the quotent projection in degree and in degree , and the right one being in degree , the image inclusion in degree and the identity in degree ).
For this is discussed in (BFGM).
The homotopy groups of a strict -groupoid in any degree are simply given by the groups of -automorphisms of the identity -morphism on a given baspoint modulo -morphisms (hence the homology of the corresponding crossed complex in that degree). Therefore it is clear from the construction of above that is surjective on and an isomorphism on , and that is a monomorphism on and an isomorphism on .
It is known that Postnikov systems classify all weak, pointed connected homotopy types. In particular, if satisfies for then the first non trivial Postnikov invariant is an element of group cohomology (with twisted coefficients of course). Such elements are also determined by -fold crossed extensions of by , which are exact crossed complexes of the form
together with an isomorphism . This gives an algebraic model of such an -type. Advantages of algebraic models are that algebraic constructions can be made on them, such as forming limits or colimits. The various higher homotopy van Kampen theorems are useful in the latter case. For example, it may be difficult or well nigh impossible to write down a determination of the Postnikov invariant of a pushout of crossed modules, even if the pushout consists of finite groups.
A Postnikov system is easiest to understand in the 2-stage case, i.e. two non vanishing homotopy groups, and focuses attention on the cohomology of Eilenberg-Mac Lane spaces, which also determine all cohomology operations. Basic work on this area was done by Eilenberg and Mac Lane, and by H. Cartan, while the theory of cohomology operations, including Steenrod operations, is itself a large area.
The reference below shows the problems in the 3-stage systems.
For homotopy 3-types, the algebraic model of crossed squares is more explicit than the corresponding Postnikov system, and more calculable. However, not much work has been done on, say, cohomology operations using the algebraic model of -fold groupoids, and it is not clear if that would help.
For simplicial sets
Let be a Kan complex and the model for its Postnikov tower from prop. 2. For any vertex write for the pullback
Let be the Eilenberg-MacLane object on the -homotopy group of . Then there is a weak homotopy equivalence
This appears for instance as GoerssJardine, corollary VI 3.7.
Since is a Kan fibration by prop. 2 the pullback is the homotopy fiber of .
There are analogues in other setups, e.g.
Postnikov tower in an -category
We may think of Top as being the archetypical (∞,1)-category.
In every (∞,1)-category there is a notion of n-truncated object and accordingly a notion of
The traditional case of Postnikov towers in Top is a special case of this more general concept.
A standard textbook reference is section 8 of
and section VI of
Orginal references include
M. M. Postnikov, Determination of the homology groups of a space by means of the homotopy invariants, Doklady Akad. Nauk SSSR (N.S.) 76: 359–362 (1951)
George Whitehead, Elements of homotopy theory, chapter 9
Donald W. Kahn, The spectral sequence of a Postnikov system, Comm. Math. Helv. 40, n.1, 169–198, 1965 doi
P. I. Booth, An explicit classification of three-stage Postnikov towers, Homology, homotopy and applications 8 (2006), No. 2, 133–155
G. J. Ellis and R. Mikhailov, A colimit of classifying spaces, arXiv:0804.3581.
Probably the earliest treatment of Postnikov systems for simplicial sets is in
- J. C. Moore, Semisimplicial complexes and Postnikov systems, Symposium Internacional de Topologia Algebraica, Mexico City, 1958, pp. 232-247,
and as a result, in that context, they are sometimes referred to as Moore-Postnikov systems
The coskeleton construction for the Postnikov tower of a Kan complex is already in
- Artin, Mazur, Étale homotopy, (Lecture Notes in Maths. 100).
Another classical article that amplifies the expression of Postnikov towers in terms of coskeleta is
Analogous remarks are also in
- John Duskin Simplicial matrices and the nerves of weak -categories I: Nerves of bicategories , TAC 9 no. 2, (2002). (web)
reviewed around page 302 in
- Tibor Beke, Higher Čech theory, K-Theory, 32(4):293–322 (2004) (web)
Discussion for homotopy types modeled by crossed complexes/strict ω-groupoids is in
- M. Bullejos, E. Faro, and M. A. García-Munoz, Postnikov Invariants of Crossed Complexes, Journal of Algebra Volume 285, Issue 1, 1 March 2005, Pages 238–291 (arXiv:math/0409339).
and for -hypergroupoids in the thesis,
- M. A. García-Munoz, 2003, Un aceramiento algebraico a la theoría de Torres de Postnikov, thesis, Universidad de Granada.
This also contains a good discussion of the link with twisted cohomology and homotopy colimits.
A pedagogical introduction to Postnikov systems with an eye towards their generalization from homotopy types to n-categories is in