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Pin(2)-equivariant Seiberg-Witten Floer homology of Seifert fibrations

We compute the $\mathrm{Pin}(2)$-equivariant Seiberg-Witten Floer homology of Seifert rational homology three-spheres in terms of their Heegaard Floer homology. As a result of this computation, we prove Manolescu's conjecture that $β=-\barμ$ for Seifert integral homology three-spheres. We show that the Manolescu invariants $α, β,$ and $γ$ give new obstructions to homology cobordisms between Seifert fiber spaces, and that many Seifert homology spheres $Σ(a_1,...,a_n)$ are not homology cobordant to any $-Σ(b_1,...,b_n)$. We then use the same invariants to give an example of an integral homology sphere not homology cobordant to any Seifert fiber space. We also show that the $\mathrm{Pin}(2)$-equivariant Seiberg-Witten Floer spectrum provides homology cobordism obstructions distinct from $α,β,$ and $γ$. In particular, we identify an $\mathbb{F}[U]$-module called connected Seiberg-Witten Floer homology, whose isomorphism class is a homology cobordism invariant.