Monthly Archives: February 2011

Group Theory ∞ (Epilogue)

That concludes the end of the series of notes on Group Theory. Has it been successful? I don’t know, but I’m reasonably pleased with the way the notes turn out, except the fact that there’s a huge disparity between the … Continue reading

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Group Theory XII.5 (More Category Theory)

Since a category is really a bunch of abstract objects and arrows between them, we can reverse them by duality. Let C be a category. The opposite category Cop is the category such that: Ob(Cop) = Ob(C); for any objects … Continue reading

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Group Theory XII.4 (Category Theory: Functors)

In this section, we will explore further concepts in category theory. First, we shall talk about “maps” between categories. Let C, D be categories. A (covariant) functor (written as F : C → D) is a map F : Ob(C) → … Continue reading

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Group Theory XII.3 (More Universal Properties)

In this section, we shall get more practice with universal properties for various algebraic constructions. First, take the following categories: Set = category of sets, with morphisms = set functions; Grp = category of groups, with morphisms = group homomorphisms; … Continue reading

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Group Theory XII.2 (Category Theory)

Let’s take a closer look at the proofs and definition in the previous section. What concepts have we used? We have considered groups, homomorphisms between them, composition of homomorphisms, identity homomorphisms, isomorphisms and inverse homomorphisms. But the last two items … Continue reading

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Group Theory XII.1 (Introduction to Diagrams)

Given groups G and H, recall we have the product P = G × H and projection maps π1:P → G and π2:P → H. There’s nothing mysterious about the projection maps: these just take (g, h) to g and h respectively. But … Continue reading

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Group Theory XI.4 (Nilpotent Groups: Examples etc)

We will talk more about the properties of nilpotent groups, before narrowing down to the concrete examples (as well as non-examples). The first thing we shall prove is: All p-groups are nilpotent. Proof. Let G be a p-group; its centre Z(G) is … Continue reading

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