## Modular categories after Turaev II: Invariants of 3-manifolds & the skein category

### Daniel Steffen – 15 September 1999

These talks were an exposÃ© of some definitions and results in V.G. Turaev, Quantum Invariants of Knots and 3-Manifolds, de Gruyter, 1994 (MR 95k:57014) on tortile tensor categories, modular categories and their connection to low-dimensional topology.
For V a strict tortile tensor category, an isotopy invariant F on V-colored ribbon graphs with values in V is given. V-colored ribbon graphs are a generalization of framed tangles, whose components are equipped with objects and morphisms of V.
A (quasi-)modular category is an additive strict tortile tensor category equipped with a finite family of simple objects (quasi-)dominating the category and respecting some axioms.
Considering F on such a modular category W allows its extension to a topological invariant tau of closed connected oriented 3-manifolds with embedded W-colored ribbon graphs.
For A a ribbon Hopf algebra, the category of A-modules of finite rank quasi-dominated by a finite family of simple A-modules and respecting the axioms is a quasi-modular category.
A non-trivial geometrical example of a quasi-modular category is the refined skein category.

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