Pl\"ucker Coordinates and the Rosenfeld Planes
The exceptional compact hermitian symmetric space EIII is the quotient $E_6/Spin(10)\times_{\mathbb{Z}_4}U(1)$. We introduce the Pl\"ucker coordinates which give an embedding of EIII into $\mathbb{C}P^{26}$ as a projective subvariety. The subvariety is cut out by 27 Pl\"ucker relations. We show that, using Clifford algebra, one can solve this over-determined system of relations, giving local coordinate charts to the space. Our motivation is to understand EIII as the complex projective octonion plane $(\mathbb{C}\otimes\mathbb{O})P^2$, whose construction is somewhat scattered across the literature. We will see that the EIII has an atlas whose transition functions have clear octonion interpretations, apart from those covering a sub-variety $X_{\infty}$ of dimension 10. This subvariety is itself a hermitian symmetric space known as DIII, with no apparent octonion interpretation. We give detailed analysis of the geometry in the neighbourhood of $X_{\infty}$. We further decompose $X={\rm EIII}$ into $F_4$-orbits: $X=Y_0\cup Y_{\infty}$, where $Y_0\sim(\mathbb{O}P^2)_{\mathbb{C}}$ is an open $F_4$-orbit and is the complexification of $\mathbb{O}P^2$, whereas $Y_{\infty}$ has co-dimension 1, thus EIII could be more appropriately denoted as $\overline{(\mathbb{O}P^2)_{\mathbb{C}}}$. This decomposition appears in the classification of equivariant completion of homogeneous algebraic varieties by Ahiezer \cite{Ahiezer}.