Remarks on flat $S^1$-bundles, $C^\infty$ vs $C^\omega$

Abstract

We describe low dimensional homology groups of $\mathrm{Diff}^\delta_+S^1$ in terms of Haefliger's classifying space $B\overline{\Gamma}_1$ by applying a theorem of Thurston. Then we consider the question whether some power of the rational Euler class vanishes for real analytic flat $S^1$-bundles. We show that if it occurs, then the homology group of $\mathrm{Diff}_+^{\omega,\delta} S^1$ should contain two kinds of many torsion classes which vanish in $\mathrm{Diff}^\delta_+S^1$. This is an informal note on our discussions about the above question.