The forcing flexibility conjecture for automorphism-tower heights

From papers

Let VV be the ground model. For a centreless group GVG\in V, let τ(G)\tau(G) be the least ordinal α\alpha such that the automorphism tower satisfies Gβ=GαG_\beta=G_\alpha for all β>α\beta>\alpha, and let τVP(G)\tau^{V^{\mathbb{P}}}(G) denote the value computed in the forcing extension VPV^{\mathbb{P}}. Forcing flexibility conjecture. Let α\alpha and β\beta be ordinals such that if α1\alpha\geq 1, then β1\beta\geq 1. Then there exist a centreless group GG and a notion of forcing P\mathbb{P} such that

τ(G)=αandτVP(G)=β.\tau(G)=\alpha\quad\text{and}\quad\tau^{V^{\mathbb{P}}}(G)=\beta.

This conjecture asks whether the height of an automorphism tower can be changed to any prescribed ordinal height by passing to a suitable forcing extension, subject only to the fact that a nonzero height in the ground model remains nonzero in the extension. Earlier results showed that automorphism towers of centreless groups terminate and that their heights can increase or decrease in generic extensions, but the full simultaneous realization asserted here was not known from the supplied context.

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Sources & referencesView supporting material

Primary source

Joel David Hamkins and Simon Thomas, “Changing the heights of automorphism towers”, arXiv:math/9703204 (1997).

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