## Surgery in complex geometry

I've been thinking about surgery on complex manifolds. Not very seriously, but just to the point that I think it's odd how there's almost no mention of it in the literature. I figure there's something I'm missing.

In "Complex manifolds and deformations complex structures" Kodaira defines a surgery operation which takes two complex manifolds, makes certain choices, and returns a third complex manifold obtained by joining the two along a submanifold. The details are on page 52 of Kodaira's book, which may be found here.

After a quick look it seems to me that Kodaira uses this surgery to construct the Hirzebruch surfaces, the Hopf surface, and the blow-up of a point in $\mathbb C^n$. Elsewhere, the only similar examples I can find are the ones of a blow-up of a point or subvariety on a complex manifold (see, for example, page 93 of Zheng's "Complex differential geometry").

So my question is, why don't we hear more about surgery of complex manifolds? Heuristically I'd expect that it doesn't work all that well, because else I'd already seen it used to construct (counter-)examples, but I also can't figure out why it doesn't work.

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 One reason might be that surgery theory fails miserably in any category less flabby than diff (I don't claim that the surgery fails, just the theory). The h-cobordism theorem is false for analytic manifolds, and with that goes every nontrivial result I know (admittedly not that many) in surgery theory. – Paul Siegel Dec 21 2010 at 23:54 Note that we have surgery in Ricci flow pioneered by Perelman "Ricci flow with surgery on three-manifolds".So when we want to know more about Kahler-Ricci flow,it's unavoidable to consider some surgeries on complex manifolds. – xiao Jan 2 2011 at 14:36 Perelman did surgery on smooth manifolds, not complex manifolds. From what I understand (which isn't very much) surgery was needed to show existence of the Ricci flow in three real dimensions. I believe we have existence for the Kahler-Ricci flow on compact manifolds, but I'd be surprised if the same methods were used in that proof. – Gunnar Magnusson Jan 2 2011 at 17:42

 Thank you for that Sándor. I do think there should be a completely complex analytic notion of surgery (1: like Kodaira's, 2: maybe it would coincide with flips in the projective case), because a surgery is a mostly local operation, and locally speaking we have more functions to glue with in the analytic category than in the algebraic one. Also I feel that surgery should make sense for non-algebraic manifolds, so I'd expect an operation which makes explicit use of algebraic properties to be a special case of something more general. – Gunnar Magnusson Dec 21 2010 at 20:41 Also: do we know the topology of a manifold obtained as a flip? I ask because we know the Betti numbers of a blow-up of $X$ at a point: they're the Betti numbers of $X$, plus the Betti numbers of $\mathbb P^n$. If a blow-up is a special case of surgery, then I'd expect (or at least very much like) to see some sort of relation between the topology of the manifold obtained by surgery and the original manifolds. – Gunnar Magnusson Dec 21 2010 at 20:55 @Gunnar: what about applying Mayer-Vietoris and/or Van Kampen? – Qfwfq Dec 22 2010 at 1:49 Gunnar: I totally agree that there should be a more general, complex analytic notion of which flips would be special cases. In fact, I wonder if that would perhaps lead to a new way to construct flips. In fact, I would expect that a reasonable definition of surgery would be more general than flips even in the algebraic case. As you mention blow-ups, those are not flips, so that's already one. – Sándor Kovács Dec 22 2010 at 4:54 Gunnar #2: regarding the topology of flips: I am not sure how much research has been done in this direction. Certain things are easy: anything that only depends on codimension 1 data remains the same as flips are isomorphisms in codimension 1. On the other hand, in general the topology changes and as far as I understand it is hard to control how (at least for an algebraic geometer). An extremely interesting relevant work is a series of papers of Kollár on the minimal model program over the reals. In particular, he is working very hard to not change the topology in an uncontrolled way (cont) – Sándor Kovács Dec 22 2010 at 5:00