# Bockstein homomorphism

In homological algebra, the Bockstein homomorphism, introduced by Meyer Bockstein (1942, 1943, 1958), is a connecting homomorphism associated with a short exact sequence

$0\to P\to Q\to R\to 0$ of abelian groups, when they are introduced as coefficients into a chain complex C, and which appears in the homology groups as a homomorphism reducing degree by one,

$\beta \colon H_{i}(C,R)\to H_{i-1}(C,P).$ To be more precise, C should be a complex of free, or at least torsion-free, abelian groups, and the homology is of the complexes formed by tensor product with C (some flat module condition should enter). The construction of β is by the usual argument (snake lemma).

A similar construction applies to cohomology groups, this time increasing degree by one. Thus we have

$\beta \colon H^{i}(C,R)\to H^{i+1}(C,P).$ The Bockstein homomorphism $\beta$ associated to the coefficient sequence

$0\to \mathbb {Z} /p\mathbb {Z} \to \mathbb {Z} /p^{2}\mathbb {Z} \to \mathbb {Z} /p\mathbb {Z} \to 0$ is used as one of the generators of the Steenrod algebra. This Bockstein homomorphism has the following two properties:

$\beta \beta =0$ if $p>2$ ,
$\beta (a\cup b)=\beta (a)\cup b+(-1)^{\dim a}a\cup \beta (b)$ ;

in other words, it is a superderivation acting on the cohomology mod p of a space.