ScalingStacks

Example 2.3 . [01Y2]

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Example 2.3.

(Infinitely many topological types in dimension 4) Let T3T^{3} denote a flat 33-torus. According to Anderson [A93], there is a collapsing sequence of manifolds (Mj4,dj)⟶dG​HT3(M^{4}_{j},d_{j})\stackrel{{\scriptstyle d_{GH}}}{{\longrightarrow}}T^{3} satisfying

diam⁡(Mj4)≤1,\displaystyle{\rm diam}(M^{4}_{j})\leq 1\,,
|RicMjn|≤ϵj→0,\displaystyle|{\rm Ric}_{M^{n}_{j}}|\leq\epsilon_{j}\to 0\,,
Vol⁡(Mj4)→0,\displaystyle{\rm Vol}(M^{4}_{j})\to 0\,,
b2​(Mj4)→∞,\displaystyle b_{2}(M^{4}_{j})\to\infty\,, (2.11)

where b2​(Mj4)b_{2}(M^{4}_{j}) denotes the second Betti number of Mj4M^{4}_{j}. In particular, Theorem 1.4 , the finiteness theorem in dimension 44, does not extend to the case in which the lower volume bound is dropped.

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