Quasitriangular Hopf algebra

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In mathematics, a Hopf algebra, H, is quasitriangular if there exists an invertible element, R, of H \otimes H such that

  • R \ \Delta(x) = (T \circ \Delta)(x) \ R for all x \in H, where Δ is the coproduct on H, and the linear map T : H \otimes H \to H \otimes H is given by T(x \otimes y) = y \otimes x,
  • (\Delta \otimes 1)(R) = R_{13} \ R_{23},
  • (1 \otimes \Delta)(R) = R_{13} \ R_{12},

where R12 = φ12(R), R13 = φ13(R), and R23 = φ23(R), where \phi_{12} : H \otimes H \to H \otimes H \otimes H, \phi_{13} : H \otimes H \to H \otimes H \otimes H, and \phi_{23} : H \otimes H \to H \otimes H \otimes H, are algebra morphisms determined by

\phi_{12}(a \otimes b) = a \otimes b \otimes 1,
\phi_{13}(a \otimes b) = a \otimes 1 \otimes b,
\phi_{23}(a \otimes b) = 1 \otimes a \otimes b.

R is called the R-matrix.

As a consequence of the properties of quasitriangularity, the R-matrix, R, is a solution of the Yang-Baxter equation (and so a module V of H can be used to determine quasi-invariants of braids, knots and links). Also as a consequence of the properties of quasitriangularity, (\epsilon \otimes 1) R = (1 \otimes \epsilon) R = 1 \in H; moreover R^{-1} = (S \otimes 1)(R), R = (1 \otimes S)(R^{-1}), and (S \otimes S)(R) = R. One may further show that the antipode S must be a linear isomorphism, and thus S^2 is an automorphism. In fact, S^2 is given by conjugating by an invertible element: S(x) = uxu − 1 where u = m (S \otimes 1)R^{21} (cf. Ribbon Hopf algebras).

It is possible to construct a quasitriangular Hopf algebra from a Hopf algebra and its dual, using the Drinfel'd quantum double construction.

[edit] Twisting

The property of being a quasi-triangular Hopf algebra is preserved by twisting via an invertible element F = \sum_i f^i \otimes f_i \in \mathcal{A \otimes A} such that (\varepsilon \otimes id )F = (id \otimes \varepsilon)F = 1 and satisfying the cocycle condition

(F \otimes 1) \circ (\Delta \otimes id) F = (1 \otimes F) \circ (id \otimes \Delta) F

Furthermore,

u = ∑ fiS(fi)
i

is invertible and the twisted antipode is given by S'(a) = uS(a)u − 1, with the twisted comultiplication, R-matrix and co-unit change according to those defined for the quasi-triangular Quasi-Hopf algebra. Such a twist is known as an admissible (or Drinfel'd) twist.

[edit] See also