Homotopy type
A class of homotopyequivalent topological spaces. Two mappings and are said to be mutuallyinverse homotopy equivalences if and . If only the first condition is met, is said to be a homotopy monomorphism and is said to be a homotopy epimorphism. A mapping is a homotopy equivalence if and only if it is both a homotopy monomorphism and a homotopy epimorphism. If there exists a homotopy epimorphism , then one says that dominates . If there exists a homotopy equivalence , then and are said to be homotopy equivalent, or spaces of the same homotopy type.
The problem of homotopy type consists in finding necessary and sufficient conditions for homotopy equivalence of arbitrary spaces. It may be convenient to weaken this statement somewhat. A mapping is said to be a weak homotopy equivalence if it induces an isomorphism of the homotopy groups in all dimensions (cf. Homotopy group). Correspondingly, two spaces and are said to be weakly homotopy equivalent if there exists either a weak homotopy equivalence or a weak homotopy equivalence . Since any homotopy equivalence is a weak homotopy equivalence, it follows that homotopyequivalent spaces are weakly homotopy equivalent. The converse is true if the spaces are CWcomplexes (Whitehead's theorem, cf. CWcomplex). This theorem is based on the facts that: 1) a mapping is a homotopy equivalence if and only if is a deformation retract of the cylinder (cf. Mapping cylinder) of ; 2) a mapping is a weak homotopy equivalence if and only if the subspace of the cylinder is homotopy representative (cf. Representative subspace); and 3) a subdivision of a complex is representative if and only if it is a deformation retract.
Thus, the problem of homotopy types on the category of complexes is equivalent with the problem of weak homotopy types. On the other hand, any space is weakly homotopy equivalent to the geometric realization of its singular simplicial set . For this reason, the problem of weak homotopy types need be considered, without restricting generality, only for complexes.
Two mappings are said to be homotopic if for any complex of dimension and any mapping the mappings and are homotopic. If is a complex, this holds if and only if . Spaces that are equivalent with respect to homotopy are called spaces of the same homotopy type. Two complexes and are said to be complexes of the same type (denoted by ) if their th skeletons and have the same homotopy type. If , then for any . This remains true even for , if the type is understood to be the homotopy type. In other words, the concept of being of type is homotopy invariant. The importance of the concept of being of type for the problem of homotopy types is due to the fact that two dimensional complexes are homotopy equivalent if and only if they have the same type.
Let be an arbitrary space (for the sake of simplicity — a totallyconnected space). A simplicial subset of the simplicial set is said to be minimal if it contains all the singular simplices that are mappings into some fixed point , and if for any simplex with all faces in there exists a unique simplex in that is homotopic to (with respect to the boundary of the standard simplex). Minimal subsets exist and, up to an isomorphism, are uniquely determined by . Moreover, two spaces are weakly homotopy equivalent if and only if their minimal simplicial sets are isomorphic. Thus, in order to solve the problem of weak homotopy type, all that remains to be done is to find a satisfactory description of the simplicial sets .
Let be a dimensional standard simplex, considered as a simplicial decomposition (with respect to its standard triangulation), and let be its dimensional cochain group over an Abelian group (more exactly, the group of normalized dimensional cochains of the simplicial set , cf. Cochain). Let be a simplicial set in which the simplices of dimension are cochains in , and the boundary operators and degeneracies are cochain mappings induced by the standard simplicial mappings and (the mapping "makes free" the th vertex, while the mapping "glues" the th and the th vertices). The simplices that are cocycles form a certain simplicial subset in . The coboundary operator defines a simplicial mapping , whose kernel is . The mapping is a fibration (in the sense of Kan) with fibre . In addition, the simplicial set is an object of type in the category of simplicial sets (with respect to homotopy groups in the sense of Kan, see Eilenberg–MacLane space), while the simplicial set is homotopy trivial (homotopy equivalent to a "point" ). Thus, the fibration is a simplicial analogue of the Serre fibration of paths over a space of type .
The simplicial set for is also meaningful for any (not necessarily Abelian) group . The simplicial set thus obtained is the standard simplicial resolution of .
Let be a multiplicative group of operators on the additive group . Let be an arbitrary simplicial set over . Let be a cocycle in . In the group of cocycles of this set over there is defined a coboundary operator with respect to . Let be an arbitrary dimensional simplex in and let be its characteristic mapping (cf. Simplicial set). This defines a cocycle in . Let the coboundary operator with respect to this cocycle be denoted by the symbol . Let be an arbitrary dimensional cocycle of the simplicial set over with respect to . If in the direct product of the simplicial sets and one considers the subset consisting of all possible pairs , , , for which , then is a simplicial subset. The formula defines a surjective mapping , which is a fibration (in the sense of Kan). This fibration will be denoted by . If the cocycle is trivial, then this fibration is identical with the fibration induced by the simplicial mapping which corresponds to the cocycle in the fibration . The fibration over an dimensional skeleton of the simplicial set has a section and is an obstruction to an extension of this section to . After identification of the simplices and it follows that . Moreover, .
Consider the following sequence of fibrations of simplicial sets:
(1) 
Its initial term is the simplicial set constructed from the multiplicative group . By definition, onedimensional simplices of are in natural bijective correspondence with the elements of . Putting such a simplex into correspondence with the corresponding element of yields some onedimensional cocycle over in . Let the cocycle be defined in by the inductive formula . The sequence (1) is called a homotopy resolvent, or a Postnikov system (the original name was: natural system), if for any the simplicial set is a set of type , where is a cocycle of dimension in over some group with respect to the cocycle (and the fibration is the fibration ). This sequence is called the resolution of the simplicial set if for any a simplicial mapping is given which is an isomorphism on and which is such that . The resolution uniquely determines the simplicial set up to an isomorphism. On the other hand, the resolution itself is uniquely determined by the groups and the cocycles . For this reason, an object , consisting of groups and cocycles , may also be called a resolution.
Not all simplicial sets have a resolution. According to the basic theorem in the theory of homotopy resolutions, a simplicial set has a resolution if and only if it is isomorphic to the minimal simplicial set of a certain topological space . In this case .
The resolution of the minimal set is constructed as follows. Let be an arbitrary dimensional simplex in . This simplex represents a mapping that maps all vertices of the simplex to the point . For this reason it defines some element of on any onedimensional boundary of . Thus appears on a onedimensional cocycle over , i.e. a dimensional simplex in ; it will be denoted by . In this way one obtains a certain (simplicial) mapping . This mapping is an isomorphism on and an epimorphism on . The next step is induction: Let the simplicial set and the simplicial mapping , which is an isomorphism on and an epimorphism on , have been already constructed for some . The mapping has a right inverse . Let be an obstruction to extension of this mapping to . The obstruction is an dimensional cocycle in over the group with respect to . For any dimensional simplex in the simplex is compatible with , and for this reason the difference element is defined (cf. Difference element in theory). Let be an arbitrary dimensional simplex in . On each dimensional boundary of the simplex it defines a certain dimensional simplex . Assigning to this face the element yields a certain dimensional cochain in over , i.e. some dimensional simplex of the simplicial set . The pair belongs to the simplicial set . To finish the induction one has to observe that the constructed mapping is a simplicial isomorphism on and an epimorphism on .
The resolution is not uniquely constructed from the set : there is freedom in the selection of the inverse mappings . The simplest way of describing this nonuniqueness is to consider resolutions in the sense of (1). In fact, two such resolutions and result from the same minimal simplicial set if and only if they are isomorphic as sequences of mappings, i.e. if for any there exists an isomorphism such that . In order to describe such an isomorphism in terms of the resolutions and it should be noted that the existence of an isomorphism is equivalent to the existence of an isomorphism of groups . Here . Furthermore, for the isomorphism there is a subsequent isomorphism if and only if there exists a isomorphism (see Operator homomorphism) and a cochain such that
(2) 
Here the isomorphism is defined by the formula
(2prm) 
Resolutions and arise from the same simplicial set if and only if there exists an isomorphism that is a isomorphism for , such that for any relation (2) holds, where is the isomorphism subsequently defined by (2prm) for , while for it is the isomorphism induced by . In this case the resolutions and are called isomorphic. The resolution of the symplicial set is called the homotopy resolution of the space . Summarizing, two spaces are weakly homotopy equivalent if and only if their homotopy resolutions are isomorphic; in particular, two complexes are homotopy equivalent if and only if their homotopy resolutions are isomorphic.
If (2) is satisfied only for , then the isomorphisms only exist for . In this situation one says that the given resolutions are isomorphic. Two complexes are of the same type if and only if their homotopy resolutions are isomorphic.
The given solution to the problem of homotopy types (or types) makes it possible to demonstrate a series of general theorems and to essentially clarify the principal sides of the subject (but an explicit computation of resolutions is only possible in a few cases). It follows that for any simplyconnected space with finite homology groups the homotopy groups can be effectively computed. An analogous statement holds for spaces whose homology groups are only finitely generated [2]. The fact that the homotopy type is completely determined by the resolution shows that any problem in homotopy theory reduces to some statement on the resolutions of the corresponding spaces. This makes it possible to classify problems by the number of cycles which form part of their solutions. If the space being studied is connected, then its resolution starts in fact with the term . If the solution of a given problem can be formulated in terms of the first nontrivial group only, this problem is called a problem of order zero (e.g. the Hopf–Whitney problem on the classification of mappings of an dimensional polyhedron into an connected space). If the groups , and the cocycle are used, the problem is called a problem of order one (e.g. the problem of classifying mappings from an dimensional polyhedron into an connected space). Similarly one defines problems of order two, three, etc. Effective solutions of problems of order zero or one are known. This is related to the fact that for any connected space the homology class of the cocycle can be effectively computed; it has the form , where is the fundamental class of the space and , for is the Steenrod operation corresponding to the natural pairing , and for some invariant of it, is known as the Pontryagin square. For problems of higher orders it is necessary to effectively compute the next cocycles . Each of these cocycles is obtained from the fundamental class by some cohomology operation of corresponding order. This, in particular, shows that the solution of any problem in homology theory can be formulated in terms of cohomology operations. However, because of the great complexity of higherorder operations, only solutions to special problems of a higher order have been obtained, using considerations of special character. Some general progress has been achieved under the assumption of stability: Under this assumption sufficiently far computation of the differentials of the Adams spectral sequence is equivalent to the computation of some stable operation of high order.
The theory of homotopy resolutions can be reformulated in the following "geometric" form. An arbitrary sequence of fibrations in the sense of Serre,
(3) 
in which each space has the property that for , is called a resolution. This sequence is called the resolution of a space if for any a mapping is defined that induces an isomorphism of the homotopy groups in dimension , and that is such that . This resolution is uniquely determined (up to an isomorphism, cf. Sequence category) by the groups and the characteristic classes of the fibration . The resolution exists for any totallyconnected space (such as the geometric realization of the "algebraic" resolution (1)) and determines this space up to weak homotopy equivalence. A fibre of the fibration is a space of type and, if is homotopy simple (cf. Homotopy group), e.g. simplyconnected, this fibration is induced by the Serre fibration of paths over the space using a mapping representing the cohomology class (cf. Eilenberg–MacLane space; Representable functor).
If the space is connected, then its resolution actually starts with . For it is convenient to regard, alongside with the "absolute" resolution (3), the resolution modulo a prime number , the definition of which differs from the definition of (3) only in that the homotopy groups are replaced by their components. If the resolutions modulo any prime number have been found for , then the determination of its "absolute" resolution presents no difficulties. Therefore, in problems of computing resolutions (including the computation of the homotopy groups) one usually restricts to resolutions "modulo" , in the computation of which the powerful methods of the theory of spectral sequences and of homology operations can be used (cf. Spectral sequence). For some spaces the computation of the resolution has been advanced sufficiently far.
For example, for the sphere (for large so that the stability condition is fulfilled) sufficiently many terms of its resolution modulo 2 are known. It is sufficient to describe the groups (i.e. the components of the group ) and the cohomology classes . The first groups have the following form:'
<tbody> </tbody>

The class has the form , where is the fundamental class. The next class has the property that on the fibre of the fibration it cuts out the class , and it is uniquely determined by it. Similarly, the class is uniquely characterized by the fact that, by reducing modulo 2, it transforms to the class . The classes and vanish and the class is uniquely characterized by the fact that on the fibre of the fibration it cuts out the class . Finally, is characterized by the fact that, by reducing modulo 2, it transforms to the class .
References
[1]  M.M. Postnikov, "Studies on the homotopy theory of continuous mappings" Trudy Mat. Inst. Steklov. , 46 (1955) (In Russian) (1. Algebraic system theory; 2. The natural system and homotopy theory) 
[2]  E.H. Brown, "Finite computability of Postnikov complexes" Ann. of Math. (2) , 65 (1957) pp. 1–20 
[3]  R.E. Mosher, M.C. Tangora, "Cohomology operations and their applications in homotopy theory" , Harper & Row (1968) pp. Chapt. 13 
Comments
The article above concentrates on one aspect of homotopy types, viz. Postnikov towers (or Postnikov decompositions) and gives a great deal of detail concerning the state of affairs in the 1950's.
According to [a4] one should distinguish three directions in homotopy theory: suspension theory, which separates stable phenomena from those which are not (cf. Suspension, and also Cohomology operation and Homotopy group), explicit geometric constructions such as Whitehead products (cf. Whitehead product) and the method of killing homotopy groups. The Postnikov tower arises by killing successively homotopy groups above a given dimension. The "dual" construction involves killing at each stage all the homotopy groups below a given dimension. This yields the Whitehead tower
where is connected, i.e. for all , the fibration has as fibre the Eilenberg–MacLane space (where ), and where above dimension the homotopy groups of and agree.
In contrast, the Postnikov approximation (Postnikov tower) of a connected CWcomplex is a sequence of fibrations
with the fibres of equal to , , and such that induces an isomorphism of homotopy groups in dimensions . Both towers can be used to calculate homotopy groups, cf. [a6] for some examples.
Closely related to killing homotopy groups are the ideas of obstruction theory [a5], cf. also Obstruction and Postnikov system.
To the three directions mentioned above there should now certainly be added the rational homotopy theory, developed by D. Sullivan based on "piecewiselinear differential forms" , cf. [a7], [a8] and also [a6].
References
[a1]  B. Gray, "Homotopy theory. An introduction to algebraic topology" , Acad. Press (1975) pp. 30; 157–167 
[a2]  E.H. Spanier, "Algebraic topology" , McGrawHill (1966) pp. 25; 437–444 
[a3]  G.W. Whitehead, "Elements of homotopy theory" , Springer (1978) pp. 23; 415–455 
[a4]  J.F. Adams, "Algebraic topology: a students guide" , Cambridge Univ. Press (1972) 
[a5]  H.J. Baues, "Obstruction theory" , Springer (1977) 
[a6]  R. Bott, L.W. Tu, "Differential forms in algebraic topology" , Springer (1982) pp. Chapt. I, Sect. 5 
[a7]  Ph.A. Griffiths, J.W. Morgan, "Rational homotopy theory and differential forms" , Birkhäuser (1981) 
[a8]  D. Sullivan, "Infinitesimal calculations in topology" Publ. Math. IHES , 47 (1977) pp. 269–331 
Homotopy type. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Homotopy_type&oldid=12990