Embedded Surfaces for Symplectic Circle Actions |
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Authors: | Yunhyung CHO Min Kyu KIM and Dong Youp SUH |
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Institution: | 1. Department of Mathematics Education, Sungkyunkwan University, Seoul, Republic of Korea;2. Department of Mathematics Education, Gyeongin National University of Education, San 59-12, Gyesandong, Gyeyang-gu, Incheon 407-753, Republic of Korea;3. Department of Mathematical Sciences, KAIST, 335 Gwahangno, Yu-sung Gu, Daejeon 305-701, Republic of Korea |
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Abstract: | The purpose of this article is to characterize symplectic and
Hamiltonian circle actions on symplectic manifolds in terms of
symplectic embeddings of Riemann surfaces. More precisely, it is
shown that (1) if $(M,\omega)$ admits a Hamiltonian $S^1$-action,
then there exists a two-sphere $S$ in $M$ with positive symplectic
area satisfying $\langle c_1(M,\omega), S]\rangle > 0$, and (2) if
the action is non-Hamiltonian, then there exists an $S^1$-invariant
symplectic $2$-torus $T$ in $(M,\omega)$ such that $\langle
c_1(M,\omega), T]\rangle = 0$. As applications, the authors give a
very simple proof of the following well-known theorem which was
proved by Atiyah-Bott, Lupton-Oprea, and Ono: Suppose that
$(M,\omega)$ is a smooth closed symplectic manifold satisfying
$c_1(M,\omega)=\lambda \cdot \omega]$ for some $\lambda \in \R$ and
$G$ is a compact connected Lie group acting effectively on $M$
preserving $\omega$. Then (1) if $\lambda < 0$, then $G$ must be
trivial, (2) if $\lambda=0$, then the $G$-action is non-Hamiltonian,
and (3) if $\lambda > 0$, then the $G$-action is Hamiltonian. |
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Keywords: | Symplectic geometry Hamiltonian action |
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