Groupe de symétrie | EPFL Graph Search

Le groupe de symétrie, ou groupe des isométries, d'un objet (, signal, etc.) est le groupe de toutes les isométries sous lesquelles cet objet est globalement invariant, l'opération de ce groupe étant la composition. C'est un sous-groupe du groupe euclidien, qui est le groupe des isométries de l'espace affine euclidien ambiant. (Si cela n'est pas indiqué, nous considérons ici les groupes de symétrie en géométrie euclidienne, mais le concept peut aussi être étudié dans des contextes plus larges, voir ci-dessous.) Les « objets » peuvent être des figures géométriques, des images et des motifs, tels que les motifs de papier peint. La définition peut être rendue plus précise en précisant ce que l'on entend par image ou motif, par exemple une fonction de position avec des valeurs dans un ensemble de couleurs. Pour la symétrie des corps en 3D, par exemple, on peut aussi vouloir prendre en compte la composition physique. Le groupe des isométries de l'espace induit une action de groupe sur l

Long-range magnetic ordering in Li2MnGeO4 and precursor short-range spin correlations

Artem Korshunov, Irina Safiulina

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate Li2MnGeO4 were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the Pmn2(1) space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector k = (1/2 1/2 1/2) has been found below T-N approximate to 8 K. Symmetry analysis revealed the model of the ground state spin structure within the C(a)c (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of 4.9 mu(B)/Mn2+ at 1.7 K, which corresponds to the saturated value for the high-spin configuration S = 5/2. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above T-N. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in Li2MnGeO4, which is symmetry consistent on a small scale with the long-range magnetic state below T-N. The reconstructed radial spin-pair correlation function S(0)S(r) displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at r approximate to 12 angstrom at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below T-N. As a result, an exhaustive picture of the gradual formation of magnetic ordering in Li2MnGeO4 is presented.

2020

Reductive overgroups of distinguished unipotent elements in simple algebraic groups

Mikko Tapani Korhonen

Let

G

be a simple linear algebraic group over an algebraically closed field

K

of characteristic

p \geq 0

. In this thesis, we investigate closed connected reductive subgroups

X < G

that contain a given distinguished unipotent element

u

G

. Our main result is the classification of all such

X

that are maximal among the closed connected subgroups of

G

. When

G

is simple of exceptional type, the result is easily read from the tables computed by Lawther (J. Algebra, 2009). Our focus is then on the case where

G

is simple of classical type, say

G = \operatorname{SL}(V)

G = \operatorname{Sp}(V)

, or

G = \operatorname{SO}(V)

. We begin by considering the maximal closed connected subgroups

X

G

which belong to one of the families of the so-called \emph{geometric subgroups}. Here the only difficult case is the one where

X

is the stabilizer of a tensor decomposition of

V

. For

p = 2

and

X = \operatorname{Sp}(V_1) \otimes \operatorname{Sp}(V_2)

, we solve the problem with explicit calculations; for the other tensor product subgroups we apply a result of Barry (Comm. Algebra, 2015). After the geometric subgroups, the maximal closed connected subgroups that remain are the

X < G

such that

X

is simple and

V

is an irreducible and tensor indecomposable

X

-module. The bulk of this thesis is concerned with this case. We determine all triples

(X, u, \varphi)

where

X

is a simple algebraic group,

u \in X

is a unipotent element, and

\varphi: X \rightarrow G

is a rational irreducible representation such that

\varphi(u)

is a distinguished unipotent element of

G

. When

p = 0

, this was done in previous work by Liebeck, Seitz and Testerman (Pac. J. Math, 2015). In the final chapter of the thesis, we consider the more general problem of finding all connected reductive subgroups

X

G

that contain a distinguished unipotent element

u

G

. This leads us to consider connected reductive overgroups

X

u

which are contained in some proper parabolic subgroup of

G

. Testerman and Zalesski (Proc. Am. Math. Soc, 2013) have shown that when

u

is a regular unipotent element of

G

, no such

X

exists. We give several examples which show that their result does not generalize to distinguished unipotent elements. As an extension of the Testerman-Zalesski result, we show that except for two known examples which occur in the case where

(G, p) = (C_2, 2)

, a connected reductive overgroup of a distinguished unipotent element of order

p

cannot be contained in a proper parabolic subgroup of

G

EPFL2018

Reductive overgroups of distinguished unipotent elements in simple algebraic groups

Mikko Tapani Korhonen

Let

G

be a simple linear algebraic group over an algebraically closed field

K

of characteristic

p \geq 0

. In this thesis, we investigate closed connected reductive subgroups

X < G

that contain a given distinguished unipotent element

u

G

. Our main result is the classification of all such

X

that are maximal among the closed connected subgroups of

G

. When

G

is simple of exceptional type, the result is easily read from the tables computed by Lawther (J. Algebra, 2009). Our focus is then on the case where

G

is simple of classical type, say

G = \operatorname{SL}(V)

G = \operatorname{Sp}(V)

, or

G = \operatorname{SO}(V)

. We begin by considering the maximal closed connected subgroups

X

G

which belong to one of the families of the so-called \emph{geometric subgroups}. Here the only difficult case is the one where

X

is the stabilizer of a tensor decomposition of

V

. For

p = 2

and

X = \operatorname{Sp}(V_1) \otimes \operatorname{Sp}(V_2)

X < G

such that

X

is simple and

V

is an irreducible and tensor indecomposable

X

-module. The bulk of this thesis is concerned with this case. We determine all triples

(X, u, \varphi)

where

X

is a simple algebraic group,

u \in X

is a unipotent element, and

\varphi: X \rightarrow G

is a rational irreducible representation such that

\varphi(u)

is a distinguished unipotent element of

G

. When

p = 0

X

G

that contain a distinguished unipotent element

u

G

. This leads us to consider connected reductive overgroups

X

u

which are contained in some proper parabolic subgroup of

G

. Testerman and Zalesski (Proc. Am. Math. Soc, 2013) have shown that when

u

is a regular unipotent element of

G

, no such

X

(G, p) = (C_2, 2)

, a connected reductive overgroup of a distinguished unipotent element of order

p

cannot be contained in a proper parabolic subgroup of

G

EPFL2018