# Yangian

In representation theory, a Yangian is an infinite-dimensional Hopf algebra, a type of a quantum group. Yangians first appeared in physics in the work of Ludvig Faddeev and his school in the late 1970s and early 1980s concerning the quantum inverse scattering method. The name Yangian was introduced by Vladimir Drinfeld in 1985 in honor of C.N. Yang.

Initially, they were considered a convenient tool to generate the solutions of the quantum Yang–Baxter equation.

The center of Yangian can be described by quantum determinant.

## Description

For any finite-dimensional semisimple Lie algebra a, Drinfeld defined an infinite-dimensional Hopf algebra Y(a), called the Yangian of a. This Hopf algebra is a deformation of the universal enveloping algebra U(a[z]) of the Lie algebra of polynomial loops of a given by explicit generators and relations. The relations can be encoded by identities involving a rational R-matrix. Replacing it with a trigonometric R-matrix, one arrives at affine quantum groups, defined in the same paper of Drinfeld.

In the case of the general linear Lie algebra glN, the Yangian admits a simpler description in terms of a single ternary (or RTT) relation on the matrix generators due to Faddeev and coauthors. The Yangian Y(glN) is defined to be the algebra generated by elements ${\displaystyle t_{ij}^{(p)}}$  with 1 ≤ i, jN and p ≥ 0, subject to the relations

${\displaystyle [t_{ij}^{(p+1)},t_{kl}^{(q)}]-[t_{ij}^{(p)},t_{kl}^{(q+1)}]=-(t_{kj}^{(p)}t_{il}^{(q)}-t_{kj}^{(q)}t_{il}^{(p)}).}$

Defining ${\displaystyle t_{ij}^{(-1)}=\delta _{ij}}$ , setting

${\displaystyle T(z)=\sum _{p\geq -1}t_{ij}^{(p)}z^{-p+1}}$

and introducing the R-matrix R(z) = I + z−1 P on CN${\displaystyle \otimes }$ CN, where P is the operator permuting the tensor factors, the above relations can be written more simply as the ternary relation:

${\displaystyle \displaystyle {R_{12}(z-w)T_{1}(z)T_{2}(w)=T_{2}(w)T_{1}(z)R_{12}(z-w).}}$

The Yangian becomes a Hopf algebra with comultiplication Δ, counit ε and antipode s given by

${\displaystyle (\Delta \otimes \mathrm {id} )T(z)=T_{12}(z)T_{13}(z),\,\,(\varepsilon \otimes \mathrm {id} )T(z)=I,\,\,(s\otimes \mathrm {id} )T(z)=T(z)^{-1}.}$

At special values of the spectral parameter ${\displaystyle (z-w)}$ , the R-matrix degenerates to a rank one projection. This can be used to define the quantum determinant of ${\displaystyle T(z)}$ , which generates the center of the Yangian.

The twisted Yangian Y(gl2N), introduced by G. I. Olshansky, is the co-ideal generated by the coefficients of

${\displaystyle \displaystyle {S(z)=T(z)\sigma T(-z),}}$

where σ is the involution of gl2N given by

${\displaystyle \displaystyle {\sigma (E_{ij})=(-1)^{i+j}E_{2N-j+1,2N-i+1}.}}$

Quantum determinant is the center of Yangian.

## Applications

### Classical representation theory

G.I. Olshansky and I.Cherednik discovered that the Yangian of glN is closely related with the branching properties of irreducible finite-dimensional representations of general linear algebras. In particular, the classical Gelfand–Tsetlin construction of a basis in the space of such a representation has a natural interpretation in the language of Yangians, studied by M.Nazarov and V.Tarasov. Olshansky, Nazarov and Molev later discovered a generalization of this theory to other classical Lie algebras, based on the twisted Yangian.

### Physics

The Yangian appears as a symmetry group in different models in physics.[why?]

Yangian appears as a symmetry group of one-dimensional exactly solvable models such as spin chains, Hubbard model and in models of one-dimensional relativistic quantum field theory.

The most famous occurrence is in planar supersymmetric Yang–Mills theory in four dimensions, where Yangian structures appear on the level of symmetries of operators,[1][2] and scattering amplitude as was discovered by Drummond, Henn and Plefka.

## Representation theory

Irreducible finite-dimensional representations of Yangians were parametrized by Drinfeld in a way similar to the highest weight theory in the representation theory of semisimple Lie algebras. The role of the highest weight is played by a finite set of Drinfeld polynomials. Drinfeld also discovered a generalization of the classical Schur–Weyl duality between representations of general linear and symmetric groups that involves the Yangian of slN and the degenerate affine Hecke algebra (graded Hecke algebra of type A, in George Lusztig's terminology).

Representations of Yangians have been extensively studied, but the theory is still under active development.

## Notes

1. ^ Beisert, N. (2007). The S-matrix of AdS/CFT and Yangian symmetry. arXiv preprint arXiv:0704.0400.
2. ^ Spill, F. (2009). Weakly coupled N= 4 Super Yang-Mills and N= 6 Chern-Simons theories from u (2| 2) Yangian symmetry. Journal of High Energy Physics, 2009(03), 014, https://arxiv.org/abs/0810.3897

## References

• Chari, Vyjayanthi; Andrew Pressley (1994). A Guide to Quantum Groups. Cambridge, U.K.: Cambridge University Press. ISBN 0-521-55884-0.
• Drinfeld, Vladimir Gershonovich (1985). Алгебры Хопфа и квантовое уравнение Янга-Бакстера [Hopf algebras and the quantum Yang–Baxter equation]. Doklady Akademii Nauk SSSR (in Russian). 283 (5): 1060–1064.
• Drinfeld, V. G. (1987). "A new realization of Yangians and of quantum affine algebras". Doklady Akademii Nauk SSSR (in Russian). 296 (1): 13–17. Translated in Soviet Mathematics - Doklady. 36 (2): 212–216. 1988. Missing or empty |title= (help)
• Drinfeld, V. G. (1986). Вырожденные аффинные алгебры Гекке и янгианы [Degenerate affine Hecke algebras and Yangians]. Funktsional'nyi Analiz I Ego Prilozheniya (in Russian). 20 (1): 69–70. MR 0831053. Zbl 0599.20049. Translated in Drinfeld, V. G. (1986). "Degenerate affine hecke algebras and Yangians". Functional Analysis and Its Applications. 20 (1): 58–60. doi:10.1007/BF01077318.
• Molev, Alexander Ivanovich (2007). Yangians and Classical Lie Algebras. Mathematical Surveys and Monographs. Providence, RI: American Mathematical Society. ISBN 978-0-8218-4374-1.
• Bernard, Denis (1993). "An Introduction to Yangian Symmetries". NATO ASI Series. 310 (5): 39–52. arXiv:hep-th/9211133. doi:10.1007/978-1-4899-1516-0_4. ISBN 978-1-4899-1518-4.
• MacKay, Niall (2005). "Introduction to Yangian Symmetry in Integrable Field Theory". International Journal of Modern Physics A. 20 (30): 7189–7217. arXiv:hep-th/0409183. Bibcode:2005IJMPA..20.7189M. doi:10.1142/s0217751x05022317.
• Drummond, James; Henn, Johannes; Plefka, Jan (2009). "Yangian Symmetry of Scattering Amplitudes in N = 4 super Yang-Mills Theory". Journal of High Energy Physics. 2009 (5): 046. arXiv:0902.2987. Bibcode:2009JHEP...05..046D. doi:10.1088/1126-6708/2009/05/046.