# Stable classification of 4-manifolds

## 1 Introduction


## 2 Construction and examples I

We begin with the construction of two classes of manifolds which can be used to give many stable diffeomorphism types of non-spinnable $4$$4$-manifolds. The first is:

• $\CP^2$$\CP^2$

The second is a large class of manifolds associated to certain algebraic data. Let $\pi$$\pi$ be a finitely presentable group. Then for each element $\alpha$$\alpha$ in $H_4(K(\pi,1))$$H_4(K(\pi,1))$ there is a smooth, closed, connected, oriented, non-spinnable manifold $M(\alpha)$$M(\alpha)$ with signature zero, fundamental group $\pi$$\pi$ and $u_*([M]) = \alpha$$u_*([M]) = \alpha$. This is proved in several steps by first using the Atiyah-Hirzebruch spectral sequence) and the fact that the oriented bordism groups are zero in degree $1$$1$, $2$$2$ and $3$$3$: see Oriented bordism to show that there is a closed, smooth, oriented manifold $M$$M$ together with a map $f: N \to K(\pi,1)$$f: N \to K(\pi,1)$ with $f_*([M]) = \alpha$$f_*([M]) = \alpha$ and signature zero. Then by surgeries on $0$$0$- and $1$$1$-dimensional spheres one changes $M$$M$ and $f$$f$ in such a way, that $M$$M$ is connected and $f_*$$f_*$ is an isomorphism on $\pi_1$$\pi_1$ (reference). Finally we form the connected sum with $\mathbb{CP}^2 \oplus (-\mathbb {CP}^2)$$\mathbb{CP}^2 \oplus (-\mathbb {CP}^2)$ to make sure that $M$$M$ is non-spinnable. This manifold is of course not unique but we will see that it is unique up to stable diffeomorphisms and we abbreviate it by

• $M(\alpha)$$M(\alpha)$

## 3 Invariants

The following is a complete list of invariants for the stable classification of closed, smooth oriented $4$$4$-manifolds whose universal covering is not spinnable:

• The Euler characteristic $\chi (M)$$\chi (M)$
• The signature $\sigma (M)$$\sigma (M)$
• The fundamanetal group $\pi_1(M)$$\pi_1(M)$
• The image of the fundamental class $[u_*([M])]\in H_4(K(\pi_1(M),1)/Out(\pi_1(M))$$[u_*([M])]\in H_4(K(\pi_1(M),1)/Out(\pi_1(M))$of $M$$M$.

Here $u:M \to K(\pi_1(M),1)$$u:M \to K(\pi_1(M),1)$ is a classifying map of the universal covering and $Out(\pi_1(M))$$Out(\pi_1(M))$ is the outer automorphism group which acts on the homology of $K(\pi_1(M),1)$$K(\pi_1(M),1)$.

## 4 Classification

Theorem 4.1. Let $M$$M$ and $N$$N$ be $4$$4$-dimensional compact smooth manifolds with non-spinnable universal covering. Then $M$$M$ and $N$$N$ are stably diffeomorphic if and only if the invariants above agree.

The proof of this result is an easy consequence of the general stable classification theorem ([Kreck1999], Stable classification of manifolds). Namely, the normal $1$$1$-type is $K(\pi,1) \times BSO \to BO$$K(\pi,1) \times BSO \to BO$, see Stable classification of manfifolds. Thus the $B$$B$-bordism group is $\Omega ^{SO}(K(\pi_1,1)$$\Omega ^{SO}(K(\pi_1,1)$, which by the Atiyah-Hirzebruch spectral sequence is ismorphic to $\mathbb Z \oplus H_4(K(\pi,1);\mathbb Z)$$\mathbb Z \oplus H_4(K(\pi,1);\mathbb Z)$ under the signature and the image of the fundamental class. Now the statement follows from Theorem 3.1 of Stable classification of manifolds.

## 5 Realization of the invariants

Given the Theorem above one wonders about the realization of the invariants. Partial answers are easy, but in general this is a complicated open question, where the answer is only known for special fundamental groups. In the simply connected case there are only two invariants, the Euler characteristic and the signature. The Euler characteristic of of a simply connected $4$$4$-manifold is $\ge 2$$\ge 2$ and it is $2$$2$ if and only if $M$$M$ is a homotopy sphere. Since a homotopy sphere is spinnable it cannot occur in our context. Thus the Euler characteristic is at least $3$$3$, the Euler characteristic of $\mathbb {CP}^2$$\mathbb {CP}^2$. By connected sum with copies of $\mathbb {CP}^2$$\mathbb {CP}^2$ we can achieve all values $\ge 3$$\ge 3$.

If the Euler characteristic of a simply connected closed $4$$4$-manifold is $k$$k$, the second Betti number is $b=k-2$$b=k-2$ and so the possible values of the signature are $-b \le s \le b$$-b \le s \le b$ with $s = k mod 2$$s = k mod 2$, since the Euler characteristic and the signature agree mod $2$$2$. For given $b$$b$ these values $s$$s$ are realized by $\sharp _{(s+b )/2 }\mathbb {CP}^2 \sharp_ {(s+b)2-s} (-\mathbb { CP}^2)$$\sharp _{(s+b )/2 }\mathbb {CP}^2 \sharp_ {(s+b)2-s} (-\mathbb { CP}^2)$. Thus we see that all possible values are realized and so we obtain:

Theorem 5.1. The stable diffeomorphism types of simply connected, closed, non-spinnable $4$$4$-manifolds are given by connected sums of copies $\mathbb {CP}^2$$\mathbb {CP}^2$ and $-\mathbb {CP}^2$$-\mathbb {CP}^2$.

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