Donaldson's theorem

In mathematics, and especially differential topology and gauge theory, Donaldson's theorem states that a definite intersection form of a compact, oriented, smooth manifold of dimension 4 is diagonalisable. If the intersection form is positive (negative) definite, it can be diagonalized to the identity matrix (negative identity matrix) over the integers. The original version^[1] of the theorem required the manifold to be simply connected, but it was later improved to apply to 4-manifolds with any fundamental group.^[2]

History

The theorem was proved by Simon Donaldson. This was a contribution cited for his Fields medal in 1986.

Idea of proof

Donaldson's proof utilizes the moduli space ${\displaystyle {\mathcal {M}}_{P}}$  of solutions to the anti-self-duality equations on a principal ${\displaystyle \operatorname {SU} (2)}$ -bundle ${\displaystyle P}$  over the four-manifold ${\displaystyle X}$ . By the Atiyah–Singer index theorem, the dimension of the moduli space is given by

${\displaystyle \dim {\mathcal {M}}=8k-3(1-b_{1}(X)+b_{+}(X)),}$ 

where ${\displaystyle c_{2}(P)=k}$ , ${\displaystyle b_{1}(X)}$  is the first Betti number of ${\displaystyle X}$  and ${\displaystyle b_{+}(X)}$  is the dimension of the positive-definite subspace of ${\displaystyle H_{2}(X,\mathbb {R} )}$  with respect to the intersection form. When ${\displaystyle X}$  is simply-connected with definite intersection form, possibly after changing orientation, one always has ${\displaystyle b_{1}(X)=0}$  and ${\displaystyle b_{+}(X)=0}$ . Thus taking any principal ${\displaystyle \operatorname {SU} (2)}$ -bundle with ${\displaystyle k=1}$ , one obtains a moduli space ${\displaystyle {\mathcal {M}}}$  of dimension five.

 

Cobordism given by Yang–Mills moduli space in Donaldson's theorem

This moduli space is non-compact and generically smooth, with singularities occurring only at the points corresponding to reducible connections, of which there are exactly ${\displaystyle b_{2}(X)}$  many.^[3] Results of Clifford Taubes and Karen Uhlenbeck show that whilst ${\displaystyle {\mathcal {M}}}$  is non-compact, its structure at infinity can be readily described.^[4][5][6] Namely, there is an open subset of ${\displaystyle {\mathcal {M}}}$ , say ${\displaystyle {\mathcal {M}}_{\varepsilon }}$ , such that for sufficiently small choices of parameter ${\displaystyle \varepsilon }$ , there is a diffeomorphism

${\displaystyle {\mathcal {M}}_{\varepsilon }{\xrightarrow {\quad \cong \quad }}X\times (0,\varepsilon )}$ .

The work of Taubes and Uhlenbeck essentially concerns constructing sequences of ASD connections on the four-manifold ${\displaystyle X}$  with curvature becoming infinitely concentrated at any given single point ${\displaystyle x\in X}$ . For each such point, in the limit one obtains a unique singular ASD connection, which becomes a well-defined smooth ASD connection at that point using Uhlenbeck's removable singularity theorem.^[6][3]

Donaldson observed that the singular points in the interior of ${\displaystyle {\mathcal {M}}}$  corresponding to reducible connections could also be described: they looked like cones over the complex projective plane ${\displaystyle \mathbb {CP} ^{2}}$ . Furthermore, we can count the number of such singular points. Let ${\displaystyle E}$  be the ${\displaystyle \mathbb {C} ^{2}}$ -bundle over ${\displaystyle X}$  associated to ${\displaystyle P}$  by the standard representation of ${\displaystyle SU(2)}$ . Then, reducible connections modulo gauge are in a 1-1 correspondence with splittings ${\displaystyle E=L\oplus L^{-1}}$  where ${\displaystyle L}$  is a complex line bundle over ${\displaystyle X}$ .^[3] Whenever ${\displaystyle E=L\oplus L^{-1}}$  we may compute:

${\displaystyle 1=k=c_{2}(E)=c_{2}(L\oplus L^{-1})=-Q(c_{1}(L),c_{1}(L))}$ ,

where ${\displaystyle Q}$  is the intersection form on the second cohomology of ${\displaystyle X}$ . Since line bundles over ${\displaystyle X}$  are classified by their first Chern class ${\displaystyle c_{1}(L)\in H^{2}(X;\mathbb {Z} )}$ , we get that reducible connections modulo gauge are in a 1-1 correspondence with pairs ${\displaystyle \pm \alpha \in H^{2}(X;\mathbb {Z} )}$  such that ${\displaystyle Q(\alpha ,\alpha )=-1}$ . Let the number of pairs be ${\displaystyle n(Q)}$ . An elementary argument that applies to any negative definite quadratic form over the integers tells us that ${\displaystyle n(Q)\leq {\text{rank}}(Q)}$ , with equality if and only if ${\displaystyle Q}$  is diagonalizable.^[3]

It is thus possible to compactify the moduli space as follows: First, cut off each cone at a reducible singularity and glue in a copy of ${\displaystyle \mathbb {CP} ^{2}}$ . Secondly, glue in a copy of ${\displaystyle X}$  itself at infinity. The resulting space is a cobordism between ${\displaystyle X}$  and a disjoint union of ${\displaystyle n(Q)}$  copies of ${\displaystyle \mathbb {CP} ^{2}}$  (of unknown orientations). The signature ${\displaystyle \sigma }$  of a four-manifold is a cobordism invariant. Thus, because ${\displaystyle X}$  is definite:

${\displaystyle {\text{rank}}(Q)=b_{2}(X)=\sigma (X)=\sigma (\bigsqcup n(Q)\mathbb {CP} ^{2})\leq n(Q)}$ ,

from which one concludes the intersection form of ${\displaystyle X}$  is diagonalizable.

Extensions

Michael Freedman had previously shown that any unimodular symmetric bilinear form is realized as the intersection form of some closed, oriented four-manifold. Combining this result with the Serre classification theorem and Donaldson's theorem, several interesting results can be seen:

1) Any non-diagonalizable intersection form gives rise to a four-dimensional topological manifold with no differentiable structure (so cannot be smoothed).

2) Two smooth simply-connected 4-manifolds are homeomorphic, if and only if, their intersection forms have the same rank, signature, and parity.

See also

• Unimodular lattice
• Donaldson theory
• Yang–Mills equations
• Rokhlin's theorem

Notes

1. Donaldson, S. K. (1983-01-01). "An application of gauge theory to four-dimensional topology". Journal of Differential Geometry. 18 (2). doi:10.4310/jdg/1214437665. ISSN 0022-040X.
2. Donaldson, S. K. (1987-01-01). "The orientation of Yang-Mills moduli spaces and 4-manifold topology". Journal of Differential Geometry. 26 (3). doi:10.4310/jdg/1214441485. ISSN 0022-040X. S2CID 120208733.
3. Donaldson, S. K. (1983). An application of gauge theory to four-dimensional topology. Journal of Differential Geometry, 18(2), 279-315.
4. Taubes, C. H. (1982). Self-dual Yang–Mills connections on non-self-dual 4-manifolds. Journal of Differential Geometry, 17(1), 139-170.
5. Uhlenbeck, K. K. (1982). Connections with L p bounds on curvature. Communications in Mathematical Physics, 83(1), 31-42.
6. Uhlenbeck, K. K. (1982). Removable singularities in Yang–Mills fields. Communications in Mathematical Physics, 83(1), 11-29.

References

• Donaldson, S. K. (1983), "An application of gauge theory to four-dimensional topology", Journal of Differential Geometry, 18 (2): 279–315, doi:10.4310/jdg/1214437665, MR 0710056, Zbl 0507.57010
• Donaldson, S. K.; Kronheimer, P. B. (1990), The Geometry of Four-Manifolds, Oxford Mathematical Monographs, ISBN 0-19-850269-9
• Freed, D. S.; Uhlenbeck, K. (1984), Instantons and Four-Manifolds, Springer
• Freedman, M.; Quinn, F. (1990), Topology of 4-Manifolds, Princeton University Press
• Scorpan, A. (2005), The Wild World of 4-Manifolds, American Mathematical Society