6-manifolds: 2-connected

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Contents 1 Introduction 2 Construction and examples 3 Invariants 4 Classification 5 Further discussion 5.1 Topological 2-connected 6-manifolds 5.2 Mapping class groups

1 Introduction

Let ${{Stub}} == Introduction == <wikitex>; Let $M$ be a closed smooth 2-connected 6-manifold and let $\mathcal{M}_6(0)$ denote the set of diffeomorphism classes of such manifolds. The classification $\mathcal{M}_6(0)$ was one of Smale's first applications of the [h-cobordism] theorem {{cite|Smale1962a|Corollary 1.3}}. The classification, as for [[Surface|oriented surfaces]] is strikingly simple: every 2-connected 6-manifold $M$ is diffeomorphic to a [[connected-sum]] $$ M \cong \sharp_r S^3 \times S^3$$ where by definition $\sharp_0 S^3 \times S^3 = S^6$ and in general $r$ is determined by the formula for the [[Euler characteristic]] of $M$ $$ \chi(M) = 2 - 2r.$$ * For the more general case where $H_2(M) \neq 0$, see [[6-manifolds: 1-connected|6-manifolds: 1-connected]]. </wikitex> == Construction and examples == <wikitex>; The following gives a complete list of 2-connected 6-manifolds up to diffeomorphism: * $S^6$, the standard 6-sphere. * $\sharp_b S^3 \times S^3$, the $b$-fold connected sum of $S^3 \times S^3$. </wikitex> == Invariants == <wikitex>; Suppose that $M$ is diffeomorphic to $\sharp_r(S^3 \times S^3)$ then: * $\pi_3(M) \cong H_3(M) \cong \Zz^{2r}$, * the third Betti-number of $M$ is given by $b_3(M) = 2r$, * the Euler characteristic of $M$ is given by $\chi(M) = 2 = 2r$, * the [[intersection form]] of $M$ is isomorphic to the sum of r-copies of $H_{-}(\Zz)$, the standard skew-symmetric hyperbolic form on $\Zz^2$. </wikitex> == Classification == <wikitex>; Recall that the following theorem was stated in other words in the introduction: {{beginthm|Theorem|{{cite|Smale1962a|Corolary 1.3}}}} The semi-group of 2-connected 6-manifolds is generated by $S^3 \times S^3$. {{endthm}} Hence if and $\Nn$ denotes the natural numbers we obtain a bijection $$ \mathcal{M}_6(0)\equiv \Nn,~~~[M] \mapsto \frac{1}{2}b_3(M).$$ </wikitex> == Further discussion == === Topological 2-connected 6-manifolds === <wikitex>; ... </wikitex> === Mapping class groups === <wikitex>; ... <wikitex> == References == {{#RefList:}} [[Category:Manifolds]]M$ be a closed smooth 2-connected 6-manifold and let

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$\mathcal{M}_6(0)$ denote the set of diffeomorphism classes of such manifolds. The classification

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$\mathcal{M}_6(0)$ was one of Smale's first applications of the [h-cobordism] theorem [Smale1962a, Corollary 1.3]. The classification, as for oriented surfaces is strikingly simple: every 2-connected 6-manifold $M$ is diffeomorphic to a connected-sum

where by definition $\sharp_0 S^3 \times S^3 = S^6$ and in general $r$ is determined by the formula for the Euler characteristic of $M$

• For the more general case where

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  $H_2(M) \neq 0$, see 6-manifolds: 1-connected.

2 Construction and examples

The following gives a complete list of 2-connected 6-manifolds up to diffeomorphism:

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  $S^6$, the standard 6-sphere.
• $\sharp_b S^3 \times S^3$, the $b$-fold connected sum of

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  $S^3 \times S^3$.

3 Invariants

Suppose that $M$ is diffeomorphic to $\sharp_r(S^3 \times S^3)$ then:

• $\pi_3(M) \cong H_3(M) \cong \Zz^{2r}$,
• the third Betti-number of $M$ is given by $b_3(M) = 2r$,
• the Euler characteristic of $M$ is given by $\chi(M) = 2 = 2r$,
• the intersection form of $M$ is isomorphic to the sum of r-copies of

  Tex syntax error

  $H_{-}(\Zz)$, the standard skew-symmetric hyperbolic form on $\Zz^2$.

4 Classification

Recall that the following theorem was stated in other words in the introduction:

Hence if and $\Nn$ denotes the natural numbers we obtain a bijection

5 Further discussion

5.1 Topological 2-connected 6-manifolds

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5.2 Mapping class groups

...

References

• [Smale1962a] S. Smale, On the structure of manifolds, Amer. J. Math. 84 (1962), 387–399. MR0153022 (27 #2991) Zbl 0109.41103