Lie Bialgebras, Fields of Cohomological Dimension at Most 2 and Hilbert's Seventeenth Problem

We investigate Lie bialgebra structures on simple Lie algebras of non-split type $A$. It turns out that there are several classes of such Lie bialgebra structures, and it is possible to classify some of them. The classification is obtained using Belavin--Drinfeld cohomology sets, which are introduced in the paper. Our description is particularly detailed over fields of cohomological dimension at most two, and is related to quaternion algebras and the Brauer group. We then extend the results to certain rational function fields over real closed fields via Pfister's theory of quadratic forms and his solution to Hilbert's Seventeenth Problem.Comment: The second version is a substantial augmentation of the first, yielding a more complete picture. Comments are welcome

Quantum groups: from Kulish-Reshetikhin discovery to classification

The aim of this paper is to provide an overview of the results about classification of quantum groups that were obtained in arXiv:1303.4046 [math.QA] and arXiv:1502.00403 [math.QA].Comment: 10 page

Poisson structures compatible with the cluster algebra structure in Grassmannians

We describe all Poisson brackets compatible with the natural cluster algebra structure in the open Schubert cell of the Grassmannian $G_k(n)$ and show that any such bracket endows $G_k(n)$ with a structure of a Poisson homogeneous space with respect to the natural action of $SL_n$ equipped with an R-matrix Poisson-Lie structure. The corresponding R-matrices belong to the simplest class in the Belavin-Drinfeld classification. Moreover, every compatible Poisson structure can be obtained this way.Comment: Minor corrections: formulation of Proposition 2.2 made more precise; as a result, proofs of Proposition 2.2 and Theorem 4.3 slightly modified; a misprint in the reference list corrected; an acknowledgment adde

Fine Structure of Class Groups \cl^{(p)}\Q(\z_n) and the Kervaire--Murthy Conjectures II

There is an Mayer-Vietoris exact sequence involving the Picard group of the integer group ring $\Z C_{p^n}$ where $C_{p^n}$ is the cyclic group of order $p^n$ and $\zeta_{n-1}$ is a primitive $p^n$-th root of unity. The "unknown" part of the sequence is a group. $V_n$. $V_n$ splits as $V_n\cong V_n^+\oplus V_n^-$ and $V_n^-$ is explicitly known. $V_n^+$ is a quotient of an in some sense simpler group $\mathcal{V}_n$. In 1977 Kervaire and Murthy conjectured that for semi-regular primes $p$, V_n^+ \cong \mathcal{V}_n^+ \cong \cl^{(p)}(\Q (\zeta_{n-1}))\cong (\mathbb{Z}/p^n \mathbb{Z})^{r(p)}, where $r(p)$ is the index of regularity of $p$. Under an extra condition on the prime $p$, Ullom calculated $V_n^+$ in 1978 in terms of the Iwasawa invariant $\lambda$ as $V_n^+ \cong (\mathbb{Z}/p^n \mathbb{Z})^{r(p)}\oplus (\mathbb{Z}/p^{n-1} \mathbb{Z})^{\lambda-r(p)}$. In the previous paper we proved that for all semi-regular primes, \mathcal{V}_n^+\cong \cl^{(p)}(\Q (\zeta_{n-1})) and that these groups are isomorphic to (\mathbb{Z}/p^n \mathbb{Z})^{r_0}\oplus (\mathbb{Z}/p^{n-1} \mathbb{Z})^{r_1-r_0} \oplus \hdots \oplus (\mathbb{Z}/p \mathbb{Z})^{r_{n-1}-r_{n-2}} for a certain sequence $\{r_k\}$ (where $r_0=r(p)$). Under Ulloms extra condition it was proved that V_n^+ \cong \mathcal{V}_n^+ \cong \cl^{(p)}(\Q(\z_{n-1})) \cong (\mathbb{Z}/p^n \mathbb{Z})^{r(p)}\oplus (\mathbb{Z}/p^{n-1}\mathbb{Z})^{\lambda-r(p)}. In the present paper we prove that Ullom's extra condition is valid for all semi-regular primes and it is hence shown that the above result holds for all semi-regular primes.Comment: 7 pages, Continuation of NT/020728

Causality from dynamical symmetry: an example from local scale-invariance

Physical ageing phenomena far from equilibrium naturally lead to dynamical scaling. It has been proposed to consider the consequences of an extension to a larger Lie algebra of local scale-transformation. The best-tested applications of this are explicitly computed co-variant two-point functions which have been compared to non-equilibrium response functions in a large variety of statistical mechanics models. It is shown that the extension of the Schr\"odinger Lie algebra $\mathfrak{sch}(1)$ to a maximal parabolic sub-algebra, when combined with a dualisation approach, is sufficient to derive the causality condition required for the interpretation of a two-point function as a physical response function. The proof is presented for the recent logarithmic extension of the differential operator representation of the Schr\"odinger algebra.Comment: 20 pages, Latex2e, 2 figures, final form (some references updated from v2

Dynamical Yang-Baxter equations, quasi-Poisson homogeneous spaces, and quantization

This paper is a continuation of [KS]. We develop the results of [KS] principally in two directions. First, we generalize the main result of [KS], the connection between the solutions of the classical dynamical Yang-Baxter equation and Poisson homogeneous spaces of Poisson Lie groups. We hope that now we present this result in its natural generality. Secondly, we propose a partial quantization of the results of [KS]. [KS] E. Karolinsky and A. Stolin, Classical dynamical r-matrices, Poisson homogeneous spaces, and Lagrangian subalgebras, Lett. Math. Phys., 60 (2002), p.257-274; e-print math.QA/0110319.Comment: 18 pages, a new section adde

Classification of Quantum Groups via Galois cohomology

The first example of a quantum group was introduced by P.~Kulish and N.~Reshetikhin. In their paper "Quantum linear problem for the sine-Gordon equation and higher representations" published in Zap. Nauchn. Sem. LOMI, 1981, Volume 101 (English version: Journal of Soviet Mathematics, 1983, 23:4), they found a new algebra which was later called $U_q (sl(2))$. Their example was developed independently by V.~Drinfeld and M.~Jimbo, which resulted in the general notion of quantum group. Recently, the so-called Belavin-Drinfeld cohomologies (twisted and untwisted) have been introduced in the literature to study and classify certain families of quantum groups and Lie bialgebras. Later, the last two authors interpreted non-twisted Belavin-Drinfeld cohomologies in terms of non-abelian Galois cohomology $H^1(\mathbb{F}, \mathbf{H})$ for a suitable algebraic $\mathbb{F}$-group $\mathbf{H}$. Here $\mathbb{F}$ is an arbitrary field of zero characteristic. The untwisted case is thus fully understood in terms of Galois cohomology. The twisted case has only been studied using Galois cohomology for the so-called ("standard") Drinfeld-Jimbo structure. The aim of the present paper is to extend these results to all twisted Belavin-Drinfeld cohomologies and thus, to present classification of quantum groups in terms of Galois cohomologies and orders. Our results show that there exist yet unknown quantum groups for Lie algebras of the types $A_n, D_{2n+1}, E_6$