Bockstein homomorphism

In mathematics, the Bockstein homomorphism in homological algebra is a connecting homomorphism associated with a short exact sequence

0 → P → Q → R → 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,

β: H_i(C, R) → H_{i − 1}(C, P).

To be more precise, C should be a complex of free abelian groups, or at least torsion free, 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

β: Hⁱ(C, R) → H^{i + 1}(C, P).

This is important as a source of cohomology operations (see Steenrod algebra). For coefficients in a finite cyclic group of order n as R, the mapping β can be combined with reduction modulo n; and then iterated.

The name is for the Soviet topologist from Moscow Meer Feliksovich Bokshtein (Bokstein), with Bockstein being a Francophone transliteration. Little known in the West, he was born October 4, 1913 and died May 2, 1990.

References

• Meyer Bockstein, Sur la formule des coefficients universels pour les groupes d'homologie C. R. Acad. Sci.Paris 247 1958 396--398.
• Edwin Spanier, Algebraic Topology