How statement and proof provenance work
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The trivial universal R-matrix for the group algebra of a two-element group
Example
Let be any field and let have basis with multiplication . Give it the Hopf structure , , and for . This is the group algebra of the cyclic group of order two. It is cocommutative, and is a triangular quasitriangular structure. The induced module map is the ordinary flip.
Facts & Assumptions
Given: A field , the two-element group with , and its displayed two-dimensional algebra and structure maps.
Hopf algebras and antipodes: a Hopf algebra is a unital algebra with algebra-map coproduct and counit satisfying the coalgebra identities, and with an antipode satisfying both convolution contractions.
The module category of a bialgebra and its tensor action: the tensor product of left -modules carries the diagonal action .
Bialgebra module tensor actions and their coherence: the canonical associator on module tensor products is -linear and the tensor action is coassociative.
Leg notation and quasitriangular Hopf algebras: is quasitriangular when it is invertible, intertwines with , and satisfies the two leg hexagons.
Braiding on the module category and triangular structures: a braiding is a natural family of module isomorphisms satisfying both tensor hexagons, and triangularity is .
Verification
Given: The field , the algebra and maps of the Example, and arbitrary left -modules when discussing the induced map.
The multiplication is associative because on basis elements with exponents modulo two; is a two-sided unit. The maps and preserve products and units on the basis , and both iterated coproducts send to , while both counit contractions return . Thus is a bialgebra.
On each basis element , both and equal . Linearity gives both antipode contractions on , so the stated structure is a Hopf algebra. Also is fixed by the flip, proving cocommutativity.
The tensor is invertible with inverse itself. For every , by cocommutativity; moreover and . Hence is quasitriangular. Since , it is triangular.
On any pair of left modules the formula reduces to . For , the two module actions agree because ; the flip is natural for module maps; and each of the two hexagon composites sends a pure triple tensor to the same reordered pure tensor. The pure tensors span, so this is the symmetric module braiding associated with .
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Sources
- Kalle Kytölä, Introduction to Hopf Algebras and Representations (Helsinki lecture notes, Spring 2011) (standard reference, not scraped)
- Pavel Etingof and Mykola Semenyakin, A Brief Introduction to Quantum Groups, arXiv:2106.05252v3 (2024) (standard reference, not scraped)