Dr. Vera Fischer (Technical University of Vienna, Austria): Vortrag: TEMPLATE ITERATIONS AND MAXIMAL COFINITARY GROUPS
Thursday, 19.03.2015 09:00 im Raum Hörsaal M6
A maximal cofinitary group is a subgroup of S∞, all of whose non-identity elements have only
finitely many fixed points and which is maximal with respect to this property under inclusion. The
minimal size of a maximal cofinitary group is denoted αg. Another well-known cardinal invariant
of the real line, denoted non(M), is the minimum size of a set of reals which is not meager. It is
a theorem of ZFC that non(M) ≤ αg. A third invariant of interest for this talk, denoted ∂, is the
minimum size of a family Ƒ of functions from ℕ to ℕ which has the property that every function
from ℕ to ℕ is eventually dominated by an element of Ƒ. In contrast with αg and non(M), ZFC does
not determine any relationship between αg and ∂. Using the method of forcing, one can show that
each of the inequalities αg < ∂ and ∂ < αg is relatively consistent with ZFC. The classical forcing
techniques seem, however, to be inadequate in addressing the latter inequality. Its consistency
was obtained only after a ground-breaking work of Shelah and the appearance of his template
iteration forcing techniques.
Jointly with A. Törnquist we have further developed these techniques to show that αg, as well as
some of its relatives, can be of countable cofinality. We define two classes of partial orders and the
iteration of arbitrary representatives of these classes along a template, which in particular provides
a uniform proof of con(cof(ā) = w) for ā ϵ {α; αp; αg; αe}. We will conclude with the discussion of
open questions and directions for further research.
Angelegt am Friday, 06.03.2015 12:49 von Julia Osthues
Geändert am Friday, 06.03.2015 12:49 von Julia Osthues
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