Spin Structures on Exceptional Flag Manifolds
Castle Rauischholzhausen – March 2016 Lie Theory and Geometry Spin and metaplectic structures on homogeneous spaces (arXiv:1602.07968) (joint work with D. V. Alekseevsky) Ioannis Chrysikos Masaryk University – Department of Mathematics and Statistics 1 • (M n, g) connected oriented pseudo-Riemannian mnfd, signature (p, q) • π : P = SO(M) → M the SOp,q-principal bundle of positively oriented orthonormal frames. n TM = SO(M) ×SOp,q R = η− ⊕ η+, n Def. (M , g) is called time-oriented (resp. space-oriented) if η− (resp. η+) is oriented. 1 • time-oriented: if and only if H (M; Z2) 3 w1(η−) = 0 1 • space-oriented: if and only if H (M; Z2) 3 w1(η+) = 0 1 • oriented: if and only if H (M; Z2) 3 w1(M) := w1(TM) = w1(η−) + w1(η+) = 0 1 Examples: ⇒ Trivial line bundle M = S ×R, w1(M) = 0, 2 1 1 2 ⇒ Torus T = S × S , w1(T ) = 0 1 1 ; Möbius strip S = S ×GR → S , G = Z2, w1(S) =6 0 w1 =6 0 w1 = 0 1 ∼ Remark: Since H (M; Z2) = Hom(π1(M); Z2) ⇒ if M is simply-connected then it is oriented 2 p,q P∞ Nr p,q • C`p,q = C`(R ) = r=0 R / < x ⊗ x − hx, xi · 1 > 0 1 • C`p,q = C`p,q + C`p,q 0 p,q 0 • Spinp,q := {x1 ··· x2k ∈ C`p,q : xj ∈ R , hxj, xji = ±1} ⊂ C`p,q • Ad : Spinp,q → SOp,q double covering n Def. A Spinp,q-structure (shortly spin structure) on (M , g) • a Spinp,q-principal bundle π˜ : Q = Spin(M) → M over M, • a Z2-cover Λ : Spin(M) → SO(M) of π : SO(M) → M, such that: Spin(M) × Spinp,q / Spin(M) π˜ Λ×Ad Λ π $ SO(M) × SOp,q / SO(M) / M • If such a pair (Q, Λ) exists, we shall call (M n, g) a pseudo-Riemannian spin manifold.
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