Spaces of Finite Subsets - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T13:24:17Z http://mathoverflow.net/feeds/question/84245 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/84245/spaces-of-finite-subsets Spaces of Finite Subsets Chris Gerig 2011-12-24T22:47:16Z 2011-12-29T11:51:40Z <p>$\exp_nX$ is the space whose underlying set is the set of nonempty subsets $S\subseteq X$ with $|S|\le n$. Its topology is the quotient one inherited from the map $X^{\oplus n}\rightarrow\exp_nX$ given by $(x_1,\ldots,x_n)\mapsto\lbrace x_1\rbrace\cup\cdots\lbrace x_n\rbrace$. And $\exp_{m\le n}X$ is canonically embedded in it.</p> <p>Interestingly, for the case $X=S^1$, we have $\exp_2S^1\approx M\ddot{o}$ (homeomorphic mobius band). Etienne Ghys saw this by considering the mobius band as $\mathbb{R}P^2$ minus the open disk (with $S^1$ as the disk's boundary) and mapping $p\in M\ddot{o}$ to the set of tangency points of the lines tangent to $S^1$ and intersecting $p$. And from this we see that $\exp_1S^1$ is the boundary of the band and not the meridian circle. Now Raul Bott showed that $\exp_3S^1\approx S^3$ (<em>On the Third Symmetric Potency of $S^1$</em>), and someone else showed that $\exp_1S^1\subset S^3$ is the trefoil knot. Furthermore, $\exp_2S^1$ is a Seifert surface of $\exp_1S^1\subset S^3$.</p> <p>My two curious questions: <em>What happens for $n\ge 3$ and the corresponding embeddings? Are there interesting results for other $X$?</em></p> http://mathoverflow.net/questions/84245/spaces-of-finite-subsets/84251#84251 Answer by Alex Suciu for Spaces of Finite Subsets Alex Suciu 2011-12-25T01:55:47Z 2011-12-26T14:03:49Z <p>The spaces $\exp_n(S^1)$, as well as the embeddings $\exp_n(S^1) \subset \exp_{n+2}(S^1)$ were studied by Christopher Tuffley in <em>Finite subset spaces of $S^1$,</em> Algebr. Geom. Topol. <strong>2</strong> (2002), 1119–1145, <a href="http://dx.doi.org/10.2140/agt.2002.2.1119" rel="nofollow">http://dx.doi.org/10.2140/agt.2002.2.1119</a>; MR1998017 (2004f:54008), and, more recently, by Sadok Kallel and Denis Sjerve in <em>Remarks on finite subset spaces</em>, Homology, Homotopy Appl. <strong>11</strong> (2009), no. 2, 229–-250, <a href="http://www.intlpress.com/hha/v11/n2/a12/" rel="nofollow">http://www.intlpress.com/hha/v11/n2/a12/</a>; MR2591920 (2011a:55019). </p> <p>In particular, based on an argument from Clifford H. Wagner's thesis (<em>Symmetric, cyclic, and permutation products of manifolds</em>, Dissertationes Math. (Rozprawy Mat.) <strong>182</strong> (1980); MR0605369 (82h:55021)), Kallel and Sjerve show that $\exp_n(S^1)$ is a closed manifold if and only if $n=1$ or $n=3$. Furthermore, Tuffley shows that $$\pi_1(\exp_{n+2}(S^1) \setminus \exp_{n}(S^1)) = \langle x, y \mid x^{n+2} = y^{n+1} \rangle.$$</p>