Past exam of the mathematics course of the University of Cambridge / 2009 / iii / Paper 16 / 5 / 4 / Solution

True: every integer occurs. The simple-connectivity assumptions in parts 1 and 2 are local to those statements and are not imposed here. For a connected closed oriented four-manifold, Poincare duality givessince $b_3=b_1$. This expression has no fixed sign or parity.

Use $\chi ({\mathbb{CP}}^2)=3$, from its three even-dimensional cells, and $\chi (S^1\times S^3)=0$, by Euler characteristic of a product. The Euler characteristic of a connected sum in dimension four isIt follows either by Mayer–Vietoris sequence after deleting balls, or by the intermediate-degree cohomology splitting and the single top and bottom generators. Therefore a connected sum of $r$ projective planes and $s$ copies of $S^1\times S^3$ hasGiven an integer $h$, choose an integer $s\ge \max \{0,\lceil (2-h)/2\rceil \}$ and put $r=h-2+2s\ge 0$. This produces a connected closed oriented four-manifold of Euler characteristic $h$. When $r=s=0$, use $S^4$. ThusIf simple connectivity were added, the possible values would instead satisfy $\chi =2+b_2\ge 2$. This distinction is recorded in Euler characteristics of closed oriented four-manifolds.