### Symmetry, Integrability and Geometry: Methods and Applications (SIGMA)

SIGMA 15 (2019), 051, 23 pages      arXiv:1811.07854      https://doi.org/10.3842/SIGMA.2019.051

### De Rham 2-Cohomology of Real Flag Manifolds

Viviana del Barco ab and Luiz Antonio Barrera San Martin b
a) UNR-CONICET, Rosario, Argentina
b) IMECC-UNICAMP, Campinas, Brazil

Received January 08, 2019, in final form June 25, 2019; Published online July 05, 2019

Abstract
Let $\mathbb{F}_{\Theta }=G/P_{\Theta }$ be a flag manifold associated to a non-compact real simple Lie group $G$ and the parabolic subgroup $P_{\Theta }$. This is a closed subgroup of $G$ determined by a subset $\Theta$ of simple restricted roots of $\mathfrak{g}=\operatorname{Lie}(G)$. This paper computes the second de Rham cohomology group of $\mathbb{F}_\Theta$. We prove that it is zero in general, with some rare exceptions. When it is non-zero, we give a basis of $H^2(\mathbb{F}_\Theta,\mathbb{R})$ through the Weil construction of closed 2-forms as characteristic forms of principal fiber bundles. The starting point is the computation of the second homology group of $\mathbb{F}_{\Theta }$ with coefficients in a ring $R$.

Key words: flag manifold; cellular homology; Schubert cell; de Rham cohomology; characteristic classes.

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