Hopf diagrams and quantum invariants.
Involutory Hopf group-coalgebras and flat bundles over 3-manifolds
Given a group π, we use involutory Hopf π-coalgebras to define a scalar invariant of flat π-bundles over 3-manifolds. When π = 1, this invariant equals the one for 3-manifolds constructed by Kuperberg from involutory Hopf algebras. We give examples which show that this invariant is non-trivial.
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