Cover (algebra)

In abstract algebra, a cover is one instance of some mathematical structure mapping onto another instance, such as a group (trivially) covering a subgroup. This should not be confused with the concept of a cover in topology.

When some object X is said to cover another object Y, the cover is given by some surjective and structure-preserving map f : X → Y. The precise meaning of "structure-preserving" depends on the kind of mathematical structure of which X and Y are instances. In order to be interesting, the cover is usually endowed with additional properties, which are highly dependent on the context.

Examples

A classic result in semigroup theory due to D. B. McAlister states that every inverse semigroup has an E-unitary cover; besides being surjective, the homomorphism in this case is also idempotent separating, meaning that in its kernel an idempotent and non-idempotent never belong to the same equivalence class.; something slightly stronger has actually be shown for inverse semigroups: every inverse semigroup admits an F-inverse cover.^[1] McAlister's covering theorem generalizes to orthodox semigroups: every orthodox semigroup has a unitary cover.^[2]

Examples from other areas of algebra include the Frattini cover^[3] and the universal cover of a Lie group.

See also

• Embedding

Notes

1. ^ Lawson p. 230
2. ^ Grilett p. 360
3. ^ Michael D. Fried, Moshe Jarden , Field arithmetic, 2nd ed, p. 506

References

• Howie, John M. (1995). Fundamentals of Semigroup Theory. Clarendon Press. ISBN 0-19-851194-9. 

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