Reference for the proof of this statement? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-18T21:19:39Z http://mathoverflow.net/feeds/question/43269 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/43269/reference-for-the-proof-of-this-statement Reference for the proof of this statement? HJ 2010-10-23T07:26:47Z 2010-10-23T09:55:41Z <p>Can anyone give me the reference for this statement?:</p> <p>Let $M$ be a closed oriented smooth 4-manifold. Any element $a\in H_2(M)$ can be represented by a smoothly embedded, oriented surface.</p> <p>I found this statement and the proof at Saveliev's book, Lectures on the topology of 3-manifold, but I think it is not a complete proof and I couldn't fill the gap. Let me know other reference.</p> http://mathoverflow.net/questions/43269/reference-for-the-proof-of-this-statement/43271#43271 Answer by Francesco Polizzi for Reference for the proof of this statement? Francesco Polizzi 2010-10-23T08:13:51Z 2010-10-23T08:34:47Z <p>By Poincaré duality, there is an isomorphism </p> <p>$H_2(M, \mathbb{Z}) \cong H^2(M, \mathbb{Z})$.</p> <p>Now let $PD(a) \in H^2(M, \mathbb{Z})$ be the Poincaré dual of $a$. Since $H^2(M, \mathbb{Z})$ classifies line bundles on $M$, there exists a line bundle $L$ such that $c_1(L)=PD(a)$. Take a general smooth section of $L$. Then its zero set is a smoothly embedded oriented surface $\Sigma \subset M$ such that its fundamental class $[\Sigma]$ is equal to $a$.</p> <p>See Donaldson-Kronheimer ["The geometry of 4-manifolds", Chapter 1] for more details.</p> http://mathoverflow.net/questions/43269/reference-for-the-proof-of-this-statement/43278#43278 Answer by Bruno Martelli for Reference for the proof of this statement? Bruno Martelli 2010-10-23T09:46:34Z 2010-10-23T09:55:41Z <p>This result actually holds in all dimensions.</p> <blockquote> <p>Let $M^n$ be a closed smooth manifold $M^n$ of any dimension $n\geqslant 3$. Every element $\alpha \in H_2(M,\mathbb Z)$ is represented by a smoothly embedded closed surface. </p> </blockquote> <p>You can prove it as follows: </p> <ol> <li>Take a cycle $a_1\sigma_1 + \ldots + a_n\sigma_n$ representing $\alpha$. The coefficients $a_i$ are integers: by writing $a\sigma$ as $\pm(\sigma +\ldots +\sigma)$ you can suppose they are all $\pm 1$. </li> <li>Since it is a cycle, restrictions on edges must cancel in pairs. You can glue correspondingly the triangles along these edges and get a map $f:S\to M^n$ from some (possibly disconnected) 2-dimensional complex $S$.</li> <li>This complex $S$ is obtained from finitely many oriented triangles by gluing the edges in pairs (with orientation-reversing maps): it is necessarily a closed oriented surface (note: this is not true for higher-dimensional cycles where you only get onlu a kind of "pseudo-manifold" which might have singular codimension-2 stratum). </li> <li>Up to homotopy you can take $f$ smooth. You can then put the map $f$ in general position. If the dimension $n$ of $M^n$ is $n\geqslant 5$ then $f$ is necessarily injective and you are done.</li> <li>IF $n=4$ you may have isolated double points. These can be removed via some surgery which replaces locally the two intersecting transverse 2-discs with an annulus. The surgery <i> modifies </i> $S$ (genus increases by one) but not the class $\alpha$.</li> <li>If $n=3$ you may have double and triple points. These can also be removed via some similar surgery.</li> </ol> <p>For a reference, I suggest you the nice and readable book "<i>The wild world of 4-manifolds</i>" from A. Scorpan which treats the 4-dimensional case and also the general $n$-dimensional one (with further discussion and references inside concerning the general problem of realizing an integral class by a manifold).</p>