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In physics, the Yang–Baxter equation (or star-triangle relation) is a consistency equation which was first introduced in the field of statistical mechanics. It depends on the idea that in some scattering situations, particles may preserve their momentum while changing their quantum internal states. It states that a matrix , acting on two out of three objects, satisfies
In one dimensional quantum systems, is the scattering matrix and if it satisfies the Yang–Baxter equation then the system is integrable. The Yang–Baxter equation also shows up when discussing knot theory and the braid groups where corresponds to swapping two strands. Since one can swap three strands two different ways, the Yang–Baxter equation enforces that both paths are the same.
Parameter-dependent Yang–Baxter equation
Let be a unital associative algebra. The parameter-dependent Yang–Baxter equation is an equation for , a parameter-dependent invertible element of the tensor product (here, is the parameter, which usually ranges over real numbers ℝ in the case of an additive parameter, or over positive real numbers ℝ+ in the case of a multiplicative parameter).
Let for , with algebra homomorphisms determined by
The Yang–Baxter equation for additive parameters is
for all values of and . For a multiplicative parameter, the Yang–Baxter equation is
for all values of and .
In some cases, the determinant of can vanish at specific values of the spectral parameter . Some matrices turn into a one dimensional projector at . In this case a quantum determinant can be defined[clarification needed].
Parameter-independent Yang–Baxter equation
Let be a unital associative algebra. The parameter-independent Yang–Baxter equation is an equation for , an invertible element of the tensor product . The Yang–Baxter equation is
where , , and .
Let be a module of . Let be the linear map satisfying for all . Then a representation of the braid group, , can be constructed on by for , where on . This representation can be used to determine quasi-invariants of braids, knots and links.
- H.-D. Doebner, J.-D. Hennig, eds, Quantum groups, Proceedings of the 8th International Workshop on Mathematical Physics, Arnold Sommerfeld Institute, Clausthal, FRG, 1989, Springer-Verlag Berlin, ISBN 3-540-53503-9.
- Vyjayanthi Chari and Andrew Pressley, A Guide to Quantum Groups, (1994), Cambridge University Press, Cambridge ISBN 0-521-55884-0.
- Jacques H.H. Perk and Helen Au-Yang, "Yang–Baxter Equations", (2006), arXiv:math-ph/0606053.