Ivanov A. Complex invariant Einstein metrics on flag manifolds with T-root system BC 2 (abstract)

Speaker: Aleksei Ivanov

Title: Complex invariant Einstein metrics on flag manifolds with $T$ -root system $BC_2$

Abstract:
Complex $G$ -invariant metrics on flag manifold $M=G/H$ are parameterised by algebraic torus $(\mathbb{C}^{*})^n$ and corresponding Einstein equations have form of Laurent polynomial equations. So we can consider Newton polytope $P$ of Einstein system which depends only on $T$ -root system of manifold $M$ . Hence, according to Bernstein-Kushnirenko theorem, the number $E(M)$ of isolated complex invariant Einstein metrics (up to multiplication on complex number) on $M$ is no greater than the normalized volume $Vol(P)$ of Newton polytope $P$ . Moreover, the equality $E(M)=Vol(P)$ holds only when Einstein system restricted on every face has no solutions in algebraic torus (also it implies that all solutions are isolated). On the other hand, if there exists a face of polytope $P$ such that Einstein system restricted on it has solution then this solution can be interpreted as a complex Ricci-flat invariant metric on some non-compact homogeneous manifold (called Inonu-Wigner contraction) associated with this face. In the talk the series of flag manifolds $M_{n_1,n_2,n_3}=SO_{2(n_1+n_2+n_3)+1}/U_{n_1} \times U_{n_2} \times SO_{2n_3+1}$ with $T$ -root system $BC_2$ will be considered. The faces of corresponding 5-dimensional Newton polytope $P(BC_2)$ were described by M. M. Graev. Using this description it will be shown that the number of complex invariant Einstein metrics on $M_{n_1,n_2,n_3}$ is equal to exactly $Vol(P)=132$ unless the parameters $n_1, n_2, n_3$ satisfy one of some algebraic equations which will be provided explicitly. Moreover, the family of (real) Ricci-flat Lorentzian manifolds will be constructed as Inonu-Wigner contractions of $M_{n_1,n_2,n_3}$ by certain 2-dimensional faces of $P(BC_2)$ .