# Steenrod-Eilenberg axioms

Axioms describing the basic properties of homology (cohomology) groups (cf. Cohomology group; Homology group), which uniquely define the relevant homology (cohomology) theory. An axiomatic homology theory is defined on a certain category of pairs of topological spaces if for any integer an Abelian group (or module over some ring) is assigned to every pair , while a homomorphism is assigned to each mapping in such a way that the following axioms are satisfied:

1) is the identity isomorphism if is the identity homeomorphism;

2) , where ;

3) connecting homomorphisms are defined such that (here , is the empty set, while the mapping , induced by , is also denoted by );

4) the exactness axiom: The homology sequence

where , are inclusions, is exact, i.e. the kernel of every homomorphism coincides with the image of the previous one;

5) the homotopy axiom: for homotopic mappings in the category under consideration;

6) the excision axiom: If the closure in of an open subset in is contained in the interior of , and the inclusion belongs to the category, then is an isomorphism;

7) the dimension axiom: when for any singleton . The group is usually called the coefficient group. Axiomatic cohomology theories are dually defined (homomorphisms are assigned to mappings ; the connecting homomorphisms take the form ). In the category of compact polyhedra, the ordinary homology and cohomology theories are the unique axiomatic theories with a given coefficient group (the uniqueness theorem). In the category of all polyhedra, the uniqueness theorem holds when the requirement is added that the homology (cohomology) of a union of open-closed, pairwise-disjoint subspaces be naturally isomorphic to the direct sum of the homology (direct product of the cohomology) of the subspaces (Milnor's additivity axiom). An axiomatic description of homology and cohomology theory also exists in more general categories of topological spaces (see [2], [3]). Generalized cohomology theories satisfy all the Steenrod–Eilenberg axioms (except for the dimension axiom), but are not uniquely defined by them.

#### References

 [1] S. Eilenberg, N.E. Steenrod, "Foundations of algebraic topology" , Princeton Univ. Press (1966) [2] S.V. Petkova, "On the axioms of homology theory" Math. USSR Sb. , 19 : 4 (1973) pp. 597–614 Mat. Sb. , 90 : 4 (1973) pp. 607–624 [3] W.S. Massey, "Notes on homology and cohomology theory" , Yale Univ. Press (1964)