No Arabic abstract
Given a symmetric operad $mathcal{P}$ and a $mathcal{P}$-algebra $V$, the associative universal enveloping algebra ${mathsf{U}_{mathcal{P}}}$ is an associative algebra whose category of modules is isomorphic to the abelian category of $V$-modules. We study the notion of PBW property for universal enveloping algebras over an operad. In case $mathcal{P}$ is Koszul a criterion for the PBW property is found. A necessary condition on the Hilbert series for $mathcal{P}$ is discovered. Moreover, given any symmetric operad $mathcal{P}$, together with a Grobner basis $G$, a condition is given in terms of the structure of the underlying trees associated with leading monomials of $G$, sufficient for the PBW property to hold. Examples are provided.
We develop an elementary method for proving the PBW theorem for associative algebras with an ascending filtration. The idea is roughly the following. At first, we deduce a proof of the PBW property for the {it ascending} filtration (with the filtered degree equal to the total degree in $x_i$s) to a suitable PBW-like property for the {it descending} filtration (with the filtered degree equal to the power of a polynomial parameter $hbar$, introduced to the problem). This PBW property for the descending filtration guarantees the genuine PBW property for the ascending filtration, for almost all specializations of the parameter $hbar$. At second, we develop some very constructive method for proving this PBW-like property for the descending filtration by powers of $hbar$, emphasizing its integrability nature. We show how the method works in three examples. As a first example, we give a proof of the classical Poincar{e}-Birkhoff-Witt theorem for Lie algebras. As a second, much less trivial example, we present a new proof of a result of Etingof and Ginzburg [EG] on PBW property of algebras with a cyclic non-commutative potential in three variables. Finally, as a third example, we found a criterium, for a general quadratic algebra which is the quotient-algebra of $T(V)[hbar]$ by the two-sided ideal, generated by $(x_iotimes x_j-x_jotimes x_i-hbarphi_{ij})_{i,j}$, with $phi_{ij}$ general quadratic non-commutative polynomials, to be a PBW for generic specialization $hbar=a$. This result seems to be new.
Universal enveloping algebras of braided m-Lie algebras and PBW theorem are obtained by means of combinatorics on words.
Consider any representation $phi$ of a finite-dimensional Lie algebra $g$ by derivations of the completed symmetric algebra $hat{S}(g^*)$ of its dual. Consider the tensor product of $hat{S}(g^*)$ and the exterior algebra $Lambda(g)$. We show that the representation $phi$ extends canonically to the representation $tildephi$ of that tensor product algebra. We construct an exterior derivative on that algebra, giving rise to a twisted version of the exterior differential calculus with the enveloping algebra in the role of the coordinate algebra. In this twisted version, the commutators between the noncommutative differentials and coordinates are formal power series in partial derivatives. The square of the corresponding exterior derivative is zero like in the classical case, but the Leibniz rule is deformed.
This paper continues the study of the lower central series quotients of an associative algebra A, regarded as a Lie algebra, which was started in math/0610410 by Feigin and Shoikhet. Namely, it provides a basis for the second quotient in the case when A is the free algebra in n generators (note that the Hilbert series of this quotient was determined earlier in math/0610410). Further, it uses this basis to determine the structure of the second quotient in the case when A is the free algebra modulo the relations saying that the generators have given nilpotency orders. Finally, it determines the structure of the third and fourth quotient in the case of 2 generators, confirming an answer conjectured in math/0610410. Finally, in the appendix, the results of math/0610410 are generalized to the case when A is an arbitrary associative algebra (under certain conditions on $A$).
We introduce a new quantized enveloping superalgebra $mathfrak{U}_q{mathfrak{p}}_n$ attached to the Lie superalgebra ${mathfrak{p}}_n$ of type $P$. The superalgebra $mathfrak{U}_q{mathfrak{p}}_n$ is a quantization of a Lie bisuperalgebra structure on ${mathfrak{p}}_n$ and we study some of its basic properties. We also introduce the periplectic $q$-Brauer algebra and prove that it is the centralizer of the $mathfrak{U}_q {mathfrak{p}}_n$-module structure on ${mathbb C}(n|n)^{otimes l}$. We end by proposing a definition for a new periplectic $q$-Schur superalgebra.