673 research outputs found
The bar involution for quantum symmetric pairs
We construct a bar involution for quantum symmetric pair coideal subalgebras
corresponding to involutive automorphisms of the
second kind of symmetrizable Kac-Moody algebras. To this end we give unified
presentations of these algebras in terms of generators and relations extending
previous results by G. Letzter and the second named author. We specify
precisely the set of parameters for which such an intrinsic bar
involution exists.Comment: Minor revision: proof of Proposition 2.3 expanded; added new Remarks
2.8, 3.1, and 3.14; simplified base field in Section 3.3; typos corrected;
final version; 25 page
Radiation hydrodynamics integrated in the code PLUTO
The transport of energy through radiation is very important in many
astrophysical phenomena. In dynamical problems the time-dependent equations of
radiation hydrodynamics have to be solved. We present a newly developed
radiation-hydrodynamics module specifically designed for the versatile MHD code
PLUTO. The solver is based on the flux-limited diffusion approximation in the
two-temperature approach. All equations are solved in the co-moving frame in
the frequency independent (grey) approximation. The hydrodynamics is solved by
the different Godunov schemes implemented in PLUTO, and for the radiation
transport we use a fully implicit scheme. The resulting system of linear
equations is solved either using the successive over-relaxation (SOR) method
(for testing purposes), or matrix solvers that are available in the PETSc
library. We state in detail the methodology and describe several test cases in
order to verify the correctness of our implementation. The solver works in
standard coordinate systems, such as Cartesian, cylindrical and spherical, and
also for non-equidistant grids. We have presented a new radiation-hydrodynamics
solver coupled to the MHD-code \PLUTO that is a modern, versatile and efficient
new module for treating complex radiation hydrodynamical problems in
astrophysics. As test cases, either purely radiative situations, or full
radiation-hydrodynamical setups (including radiative shocks and convection in
accretion discs) have been studied successfully. The new module scales very
well on parallel computers using MPI. For problems in star or planet formation,
we have added the possibility of irradiation by a central source.Comment: 13 pages, 11 figures, accepted by Astronomy & Astrophysic
Braid group actions on coideal subalgebras of quantized enveloping algebras
We construct braid group actions on coideal subalgebras of quantized
enveloping algebras which appear in the theory of quantum symmetric pairs. In
particular, we construct an action of the semidirect product of Z^n and the
classical braid group in n strands on the coideal subalgebra corresponding to
the symmetric pair (sl_{2n}(C), sp_{2n}(C)). This proves a conjecture by Molev
and Ragoucy. We expect similar actions to exist for all symmetric Lie algebras.
The given actions are inspired by Lusztig's braid group action on quantized
enveloping algebras and are defined explicitly on generators. Braid group and
algebra relations are verified with the help of the package Quagroup within the
computer algebra program GAP.Comment: 22 page
- …