Cohomology operation
A natural transformation of certain cohomology functors into others (most often — into themselves). By a cohomology operation of type ,
and
being integers and
Abelian groups, one means a family of mappings (not necessarily homomorphisms) between cohomology groups
, defined for any space
, such that
for any continuous mapping
(naturality). The set of all cohomology operations of type
forms an Abelian group with respect to the composition:
, and is denoted by
.
Examples of cohomology operations. The Steenrod reduced powers and
(cf. Steenrod reduced power); the Pontryagin square
; the Postnikov square; raising to the
-th power,
: for
, where
is a ring,
,
; the Bockstein homomorphism
; cohomology operations induced by homomorphisms
of the coefficient groups, for example,
.
Cohomology operations represent an additional structure in cohomology functors, and for this reason they make it possible to solve problems of homotopic topology that are not solvable "at the level" of cohomology groups. Examples. 1) Let and
be two spaces and let
,
be two elements. Does there exist a mapping
such that
? A first sufficient condition for the absence of such an
is the absence of a homomorphism
with
. (By this method, one can prove, for example, the Brouwer theorem on fixed points.) If
exists, then the non-existence of
can be established as follows: Let
be a cohomology operation,
, with
,
. Then
, which is impossible. 2) Is the mapping:
essential? Let
. Then (for
)
,
. If there is a cohomology operation
with
, then
is essential. In this case, the operation
detects the mapping
or the element
.
There is an isomorphism of groups , where
is an Eilenberg–MacLane space, and therefore
(see Representable functor). The groups
have been computed for all
and
and any finitely generated
and
[9].
The cohomology suspension of a cohomology operation
is the mapping
given by the composition
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where is the suspension of
. For example,
,
,
. When
,
is an isomorphism. For any
,
is a group homomorphism.
By a stable cohomology operation of type and of degree
one means a set
with
and
. Such cohomology operations form an Abelian group
, isomorphic to the group
, the latter being the inverse limit of the sequence
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The group is denoted by
.
Examples of stable cohomology operations. The Steenrod powers and
(where
is a prime number), and the Bockstein homomorphism
.
If and
, then the cohomology operation
is defined. In particular, one can define the composite
of any two stable cohomology operations
and
, so that the group
is a ring;
is called the Steenrod algebra
.
Cohomology operations first emerged in the solution of the problem of the classification of mappings of an -dimensional polyhedron into an
-dimensional sphere (
in [1] and
in [2]). The classification theorem [2]: There is an exact sequence of groups:
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The extension theorem [2]: Let be an
-dimensional polyhedron and let
be its
-dimensional skeleton. A mapping
defines an element
, where
is a generator. This mapping can be extended to
if and only if
, where
for the inclusion
.
Corresponding to the cohomology operation , the mapping
induces from the standard Serre fibration
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the fibration
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Secondary cohomology operations are cohomology classes of spaces . More precisely, let
be given, where
is an Abelian group. There exists for any
with
an element
with
, where
is the mapping induced by
; the element
depends on the choice of the element
. The arbitrariness in the choice of
is determined by the inverse image
, that is, by the orbit of the action of the group
on the set
. When
and
,
and
are group homomorphisms, and, therefore, under a different choice of
the element
can change only by some element of the subgroup
of the group
. One defines the secondary cohomology operation
by setting
(the coset
is uniquely determined by the element
). Thus, the mapping
is defined on the subgroup
and takes values in the quotient space
, where
is called the indeterminacy of the cohomology operation
. An alternative terminology is that
is a partial multi-valued cohomology operation from
into
.
Secondary cohomology operations are natural in the following sense: For any and any
, one has
[3]. If
, then
for some
, so that
, and therefore
with zero indeterminacy. Here,
, where
is any cohomology operation in
such that
, and
, so that the cohomology operation
is a uni-valued cohomology operation
restricted to
.
To each secondary cohomology operation there corresponds a relation among the ordinary (primary) cohomology operations. If , then the cohomology operation
is uniquely representable in the form
with
and
. If
is such that
, then corresponding to the cohomology operation
is the same relation
. Conversely, there corresponds to any relation of the form
a set of secondary cohomology operations
, any two of which differ from each other by a primary cohomology operation defined on the kernel of
.
A more general notion of a secondary cohomology operation is obtained by starting from a set with
and the relation
(see [3]).
Example of a secondary cohomology operation. Let
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and let be a
-generator. This gives rise to a secondary cohomology operation
corresponding to the relation
. It enables one to classify mappings of
-dimensional polyhedra into the
-dimensional sphere,
. The solution of the corresponding extension problem is as follows. Let
be an
-dimensional polyhedron and let a mapping
be given, so that one has an element
,
. Then a necessary condition for an extension of
onto
is that
; if
can be extended to
, then
is defined at
. It turns out that
can be extended to
if and only if
. Furthermore,
detects the mapping
that is the composite of the suspension of the Hopf mapping
and defines a generator of the group
.
The first solution of the "Hopf invariant problem55Q25Hopf invariant problem" (on the existence of elements of Hopf invariant one) was also given by means of secondary cohomology operations [7]. For a mapping the Hopf invariant
is defined by the formula
, where
,
are generators. The oddness of
is equivalent to the condition
, where
. When
, the operation
is decomposable in the class of primary operations, that is,
, so that
can be odd only when
. But in the class of secondary operations,
is decomposable when
, and therefore
can be odd only when
.
In addition to the secondary cohomology operations there are tertiary ones, and more generally, higher cohomology operations of any order. Corresponding to a primary cohomology operation and an element
defining a secondary cohomology operation
there is a mapping
inducing from the Serre fibration over
the fibration
;
is called the space of the cohomology operation
. If one has a cohomology class
, one can construct a tertiary cohomology operation
defined on
, the indeterminacy of which is
(under suitable restrictions on the dimension). Corresponding to this cohomology operation is the relation
, where
is a primary cohomology operation,
. Inductive continuation of this process leads to the definition of an
-th order cohomology operation. In other words, given an
-th order cohomology operation
, the space of which is
, and an element
, one constructs the
-th order cohomology operation
defined on
. Moreover, the space
is the space of the fibration induced from the Serre fibration over
by the mapping
. An axiom system for higher cohomology operations is constructed in [12].
The simplest examples of higher cohomology operations are the higher Bockstein homomorphisms. Let there be given a short exact sequence of groups
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so that
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is the corresponding exact sequence. Then the homomorphism is also a Bockstein homomorphism;
. The formula
holds; corresponding to this relation is the secondary cohomology operation
. Furthermore,
, so that there is a tertiary cohomology operation
. More generally,
is the
-th order cohomology operation constructed from the relation
. Here
is defined on
. An explicit description of
emerges in the following way: Let
and let
be a cocycle with coefficients in
representing it. Then the equation
implies that there is an integral representative
of the cocycle
, the coboundary
of which is divisible by
. Then
is the cohomology class
of the cocycle
. Thus, information about the action of the higher Bockstein cohomology operations in the groups
enables one to calculate the free part and the
-component of the group
.
To each partial cohomology operation corresponds a homotopically-simple space
with a finite number of (non-trivial) homotopy groups. Conversely, one can associate with each space
of this type a cohomology operation
for which
and
are weakly homotopy equivalent,
. For example, if
is a space with two non-trivial homotopy groups
,
,
, then there is a mapping
inducing an isomorphism
. This mapping can be converted into a fibration with fibre
; this fibration is induced from the Serre fibration over
by some mapping
; the latter defines a cohomology operation
.
These considerations enable one to describe the weak homotopy type of any space by associating with it the collection of higher cohomology operations , called its
-th Postnikov factors (see Postnikov system). For example, for the sphere
,
, the first Postnikov factor is
, and the second is
.
Another important type of cohomology operations are the functional cohomology operations [3]. To define them, a mapping ( "function" ) and a cohomology operation
are given. For
,
lies in the image of the cohomology suspension and is a group homomorphism. If
is a closed imbedding (a cofibration) and
is the inclusion mapping, then the functional cohomology operation
is defined as the partial many-valued mapping
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. This cohomology operation is defined on the subgroup
of
, and its indeterminacy is the subgroup
of
. The construction of the functional cohomology operation is natural in
. Example. If for a mapping
there exists a (primary) cohomology operation
and a cohomology class
of the space
such that
is defined and
, then
is essential. Functional and secondary cohomology operations are related to each other by the Peterson–Stein formulas (see [3]), enabling one in a number of cases to reduce the computation of secondary cohomology operations to that of primary and functional cohomology operations. There also exist higher functional cohomology operations [6]. The Massey product is a construction that is analogous to the higher cohomology operations in its structure and applications.
The concept of a cohomology operation has been carried over to generalized cohomology theories. A transformation (natural with respect to
) in a generalized cohomology theory
is called a cohomology operation of type
. These cohomology operations from a group isomorphic to the group
, where
is the
-spectrum representing the theory
. The group of all stable cohomology operations is a ring
(with respect to composition), so that
is an
-module natural with respect to
. The notions of a partial and a functional cohomology operation also have analogues in generalized cohomology theories.
By means of partial cohomology operations in ordinary cohomology theory one can solve, in principle, any homotopy problem; however, the practical application of a cohomology operation of order is extremely laborious. At the same time, it often happens that a problem requiring for its solution ordinary cohomology operations of higher order can be easily solved by the application of primary cohomology operations in a suitably chosen generalized cohomology theory. For example, the "Hopf invariant problem" is easily solved by means of the Adams primary cohomology operations
in
-theory [10]. These cohomology operations, introduced in [8] for the solution of vector fields on spheres, were the first examples of cohomology operations in a generalized cohomology theory.
The algebra was calculated [4] for
, the unitary cobordism theory, and was used in the construction of a spectral sequence of Adams type, the first term which is the cohomology space of the algebra
. Information on the action of the ring
in the groups
proves to be useful in the calculation of the Atiyah–Hirzebruch spectral sequence in the theory
.
References
[1] | L.S. Pontryagin, "A classification of mappings of a three-dimensional complex into the two-dimensional sphere" Mat. Sb. , 9 (1941) pp. 331–363 (In Russian) |
[2] | N. Steenrod, "Products of cocycles and extensions of mappings" Ann. of Math. , 48 (1947) pp. 290–320 |
[3] | R.E. Mosher, M.C. Tangora, "Cohomology operations and applications in homotopy theory" , Harper & Row (1968) |
[4] | S.P. Novikov, "The methods of algebraic topology from the viewpoint of cobordism theory" Math. USSR.-Izv. , 31 (1967) pp. 827–913 Izv. Akad. Nauk SSSR Ser. Mat. (1967) pp. 855–951 |
[5] | N.E. Steenrod, "Cohomology operations and obstructions to extending continuous functions" , Colloquium Lectures , Princeton Univ. Press (1957) |
[6] | F.P. Peterson, "Functional cohomology operations" Trans. Amer. Math. Soc. , 86 (1957) pp. 197–211 |
[7] | J.F. Adams, "On the non-existence of elements of Hopf invariant one" Ann. of Math. (2) , 72 (1960) pp. 20–104 |
[8] | J.F. Adams, "Vector fields on spheres" Ann. of Math. , 75 (1962) pp. 603–632 |
[9] | H. Cartan, "Algèbres d'Eilenberg–MacLane et homotopie" , Sem. H. Cartan , 7 (1954–1955) |
[10] | M.F. Atiyah, "![]() |
[11a] | V.M. [V.M. Bukhshtaber] Buhštaber, "Modules of differentials of the Atiyah–Hirzebruch spectral sequence" Math. USSR-Sb. , 7 : 2 (1969) pp. 299–313 Mat. Sb. , 78 (1969) pp. 307–320 |
[11b] | V.M. [V.M. Bukhshtaber] Buchštaber, "Modules of differentials of the Atiyah–Hirzebruch spectral sequence II" Math. USSR-Sb. , 12 (1970) pp. 59–75 Mat. Sb. , 83 (1970) pp. 61–76 |
[12] | C.R.F. Maunder, "Cohomology operations of the ![]() |
Comments
The spectral sequence introduced in [4], often called the Adams–Novikov spectral sequence, has been the basis for much subsequent work; see [a1].
If, as is often done, the generalized cohomology theory defined by a spectrum is denoted by
, then the ring of stable cohomology operations of
is
. The action of
on
is defined by assigning
to
and
. The ring
in fact has a Hopf algebra structure. The dual Hopf algebra
is also a most useful object of equivalent power, and in fact is sometimes technically easier to work with [a4], [a5]. Cf. Generalized cohomology theories for more details and for the definition of the Hopf algebra structures on
and
. For complex cobordism
(or
) and Brown–Peterson cohomology
, the Hopf algebras
and
have interpretations in terms of the formal group laws defined by
and
, cf. [a1], [a6].
References
[a1] | D.C. Ravenel, "Complex cobordism and stable homotopy groups of spheres" , Acad. Press (1986) |
[a2] | E.H. Spanier, "Algebraic topology" , McGraw-Hill (1966) pp. 269–276; 429–432 |
[a3] | N.E. Steenrod, D.B.A. Epstein, "Cohomology operations" , Princeton Univ. Press (1962) |
[a4] | R.M. Switzer, "Algebraic topology - homotopy and homology" , Springer (1975) pp. Chapts. 17; 18 |
[a5] | J.F. Adams, "Stable homotopy and generalised homology" , Univ. Chicago Press (1974) |
[a6] | P.S. Landweber, "![]() |
Cohomology operation. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Cohomology_operation&oldid=11241