科研之友
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摘要
We show that if g is a generic (in the sense of Baire category) isometry of a generic subspace of the Urysohn metric space U, then g does not extend to a full isometry of U. The same holds for the Urysohn sphere S. Let M be a Fraisse L-structure, where L is a relational countable language and M has no algebraicity. We provide necessary and sufficient conditions for the following to hold: "For a generic substructure A of M, every automorphism f epsilon Aut(A) extends to a full automorphism (f) over tilde Aut(M)."From our analysis, a dichotomy arises and some structural results are derived that in particular apply to omega-stable Fraisse structures without algebraicity.
- 出版日期2015-9
