# Talk:4-manifold

WikiProject Mathematics (Rated B-class, High-importance)
This article is within the scope of WikiProject Mathematics, a collaborative effort to improve the coverage of Mathematics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Mathematics rating:
 B Class
 High Importance
Field: Topology

Does anyone have a reference for the following claim maid in this article. (I thought it is still open).

Take a sum of m copies of II_{1,1} and 2n copies of E_8. If m \le 2n, then "Donaldson and Furuta proved" that no smooth structure exists. Katzmik 09:23, 9 May 2007 (UTC)

This is the 10/8 theorem, theorem 2 of a paper of Donaldson, The orientation of Yang-Mills moduli spaces and 4-manifolds topology, J. Diff. Geom. 26 (1987), 397–428.. Orthografer 17:59, 30 May 2007 (UTC)
I looked up that theorem, and it does not say this. That theorem assumes the positive part has rank 1 or 2. There is a relevant theorem of Furuta, but it's not quite as strong as stated. --Dylan Thurston (talk) 20:45, 8 September 2009 (UTC)
My mistake, Furuta's theorem is precisely what's written. Reference added. --Dylan Thurston (talk) 00:20, 10 September 2009 (UTC)

Changed priority to top because our universe is at least 4 manifold! We need to understant this Daniel de França (talk) 19:21, 12 March 2008 (UTC)

Can someone give a reference about statement "For manifolds of dimension at most 6, any piecewise linear (PL) structure can be smoothed in an essentially unique way"? Omar.zanusso (talk) 15:10, 30 May 2008 (UTC)

I can give you a meta-reference: Thurston's book mentions this fact and should have some references for it. --C S (talk) 23:15, 31 July 2008 (UTC)
That book only mentions results for dimensions 3 or less. Kirby and Siebenmann's book is a better guess; however, their techniques only apply in dimensions 5 and higher, so are not useful for this article. They refer to Cerf's 1959 paper for dimensions 4 and less, but that seems to be a mistake as far as I can tell. --Dylan Thurston (talk) 20:45, 8 September 2009 (UTC)

## Classification of homeomorphic manifolds

Article currently has a huge hole in it: a discussion of the non-classifiability of 4-manifolds, the thm by Markov (more precisely, "there is no algorithm for classifying 4-D manifolds") see e.g. http://www.mathkb.com/Uwe/Forum.aspx/math/13873/4-manifold-classification for references. Current article is even misleading: right from the get-go, just a few sentences in, it starts talking about "Freedman's classification", which seems to imply that they are classifiable! Should state first, that, in general, they're not, but that there are certain special cases which are (i.e. the Freedman case). linas (talk) 17:56, 26 November 2011 (UTC)