arXiv subjects
Publications and source records attributed to Dmitry V. Alekseevsky.
We classify all compact simply connected homogeneous CR manifolds $M$ of codimension one and with non-degenerate Levi form up to CR equivalence. The classification is based on our previous results and on a description of the maximal connected compact group $G(M)$ of automorphisms of $M$. We characterize also the standard homogeneous CR manifolds as the homogeneous CR manifolds whose group $G(M)$ in not semisimple.
An explicit classification of simply connected compact homogeneous CR manifolds G/L of codimension one, with non-degenerate Levi form, is given. There are three classes of such manifolds: a) the standard CR homogeneous manifolds which are homogeneous S^1-bundles over a flag manifold F, with CR structure induced by an invariant complex structure on F; b) the Morimoto-Nagano spaces, i.e. sphere bundles $S(N)\subset TN$ of a compact rank one symmetric space N = G/H, with the CR structure induced by the natural complex structure of $TN = G^\C/H^\C$; c) the following manifolds: $SU_n/T^1\cdot SU_{n-2}$, $SU_p\times SU_q/T^1 \cdot U_{p-2}\cdot U_{q-2}$, $SU_n/T^1\cdot SU_2\cdot SU_2\cdot SU_{n-4}$, $SO_{10}/T^1\cdot SO_6$, $E_6/T^1\cdot SO_8$; these manifolds admit canonical holomorphic fibrations over a flag manifold (F,J_F) with typical fiber S(S^k), where k = 2, 3, 5, 7 or 9, respectively; the CR structure is determined by the invariant complex structure J_F on F and by an invariant CR structure on the typical fiber, depending on one complex parameter.
We classify extended Poincaré Lie super algebras and Lie algebras of any signature (p,q), that is Lie super algebras and Z_2-graded Lie algebras g = g_0 + g_1, where g_0 = so(V) + V is the (generalized) Poincaré Lie algebra of the pseudo Euclidean vector space V = R^{p,q} of signature (p,q) and g_1 = S is the spinor so(V)-module extended to a g_0-module with kernel V. The remaining super commutators {g_1,g_1} (respectively, commutators [g_1, g_1]) are defined by an so(V)-equivariant linear mapping vee^2 g_1 -> V (respectively, wedge^2 g_1 -> V). Denote by P^+(n,s) (respectively, P^-(n,s)) the vector space of all such Lie super algebras (respectively, Lie algebras), where n = p + q = dim V and s = p - q is the signature. The description of P^+-(n,s) reduces to the construction of all so(V)-invariant bilinear forms on S and to the calculation of three Z_2-valued invariants for some of them. This calculation is based on a simple explicit model of an irreducible Clifford module S for the Clifford algebra Cl_{p,q} of arbitrary signature (p,q). As a result of the classification, we obtain the numbers L^+-(n,s) = \dim P^+-(n,s) of independent Lie super algebras and algebras, which take values 0,1,2,3,4 or 6. Due to Bott periodicity, L^+-(n,s) may be considered as periodic functions with period 8 in each argument. They are invariant under the group Gamma generated by the four reflections with respect to the axes n=-2, n=2, s-1 = -2 and s-1 = 2. Moreover, the reflection (n,s) -> (-n,s) with respect to the axis s=0 interchanges L^+ and L^- : L^+(-n,s) = L^-(n,s).