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For any compact, smooth, oriented manifold $X$ of dimension $n$ we can define its Poincaré polynomial $$p_X(z)=\sum_{k\geq0}b_k(X)z^k\in \mathbb Z[z],$$ which is the generating function of Betti numbers $b_k(X)=\operatorname{rank} H_k(X)\in \mathbb N_0$. Let $$q(z)=\sum_{k\geq0}c_kz^k$$ be a polynomial with nonnegative integral coefficients $c_k\in\mathbb N_0$, satisfying the Poincaré duality condition $c_k=c_{n-k}$. Does there exist a manifold $X$ with $p_X=q$?

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    $\begingroup$ Another necessary condition: In dimension $4k+2$, the intersection pairing on $H^{2k+1}$ is non-degenerate and antisymmetric. Thus, $b_{2k+1}$ must be even if $n = 4k+2$. $\endgroup$ – Jason DeVito Apr 8 at 16:51
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Good question, which was already asked by Poincaré himself.

Here is the answer for connected (smooth, closed, oriented) 4-manifolds: Every symmetric polynomial of the form $$ q(z)= 1+ a z+ b z^2 + a z^3 + z^4 $$ is realized as the Poincaré polynomial of some 4-manifold, provided that $a, b\in {\mathbb N}_0$. Indeed, take $M_{a,b}$ equal to the connected sum of $a$ copies of $S^3\times S^1$ and $b$ copies of $CP^2$. For the computation of homology under connected sum, see this question. The end result is that $$ b_i(M\# N)=b_i(M)+ b_i(N) $$ unless $i=0$ or $i=d$, where $M, N$ are both $d$-dimensional closed oriented manifolds. (The answer given there implicitly assumes that the manifolds are closed, i.e. compact and with empty boundary.)

More generally, you can also prescribe the constant (= the highest) coefficient of the Poincaré polynomial as long as they are $=n\in {\mathbb N}$, by taking the disjoint union of $M_{a,b}$ with $n-1$ copies of $S^4$.

See also here for further obstructions in higher dimensions.

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  • $\begingroup$ Amazing, thank you! $\endgroup$ – El Rafu Apr 8 at 20:07
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Any connected, compact, orientable 2-dimensional manifold is a genus $g$ surface. Its Poincaré polynomial is $1+2gz+z^2$. So quadratic polynomials where the coefficient of $z$ is odd cannot be the Poincaré polynomial of any compact, orienteable 2-manifold.

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    $\begingroup$ I see, thanks! Are you aware of such constraints for instance in $d=4$? $\endgroup$ – El Rafu Apr 7 at 14:51

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