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The trivial universal R-matrix for the group algebra of a two-element group

Example

Let k be any field and let H have basis 1,g with multiplication g2=1. Give it the Hopf structure Δ(gi)=gi⊗gi, ε(gi)=1, and S(gi)=gi for i∈{0,1}. This is the group algebra of the cyclic group of order two. It is cocommutative, and R=1H⊗1H is a triangular quasitriangular structure. The induced module map is the ordinary flip.

Facts & Assumptions

Given: A field k, the two-element group {1,g} with g2=1, and its displayed two-dimensional algebra and structure maps.

[F1]

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.

[F2]

The module category of a bialgebra and its tensor action: the tensor product of left H-modules carries the diagonal action h(v⊗w)=∑h(1)v⊗h(2)w.

[F3]

Bialgebra module tensor actions and their coherence: the canonical associator on module tensor products is H-linear and the tensor action is coassociative.

[F4]

Leg notation and quasitriangular Hopf algebras: R is quasitriangular when it is invertible, intertwines Δ with Δop, and satisfies the two leg hexagons.

[F5]

Braiding on the module category and triangular structures: a braiding is a natural family of module isomorphisms satisfying both tensor hexagons, and triangularity is R21R=1⊗1.

Verification

Given: The field k, the algebra H and maps of the Example, and arbitrary left H-modules V,W when discussing the induced map.

1.1givenF1construct

The multiplication is associative because on basis elements (gigj)gm=gi+j+m=gi(gjgm) with exponents modulo two; 1=g0 is a two-sided unit. The maps Δ and ε preserve products and units on the basis g0,g1, and both iterated coproducts send gi to gi⊗gi⊗gi, while both counit contractions return gi. Thus H is a bialgebra.

2.1givenF1algebrastep 1.1

On each basis element gi, both m(S⊗id⁡)Δ(gi) and m(id⁡⊗S)Δ(gi) equal g2i=1=ε(gi)1. Linearity gives both antipode contractions on H, so the stated structure is a Hopf algebra. Also Δ(gi)=gi⊗gi is fixed by the flip, proving cocommutativity.

3.1givenF4algebrastep 2.1

The tensor R=1H⊗1H is invertible with inverse itself. For every h∈H, RΔ(h)=Δ(h)=Δop(h)R by cocommutativity; moreover (Δ⊗id⁡)R=1H⊗3=R13R23 and α−1(id⁡⊗Δ)R=1H⊗3=R13R12. Hence R is quasitriangular. Since R21R=1H⊗1H, it is triangular.

4.1givenF2F3F5algebrastep 3.1∎

On any pair of left modules the formula cV,W=τV,W(ρV⊗ρW)(R) reduces to v⊗w↦w⊗v. For h∈H, the two module actions agree because Δ(h)=Δop(h); 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 R.

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