Presented by:

YM Kimchi Technion - Israel Institute of Technology

Date:

Wednesday 26th August 2015 - 14:00 to 14:30

Venue:

INI Seminar Room 2

Abstract:

Preamble: In this work we deal with partition relations
with infinite exponents under $ZFC$, hence all results are limited
to definable functions.
In [78], M. Spector has proven, basically, that
$\exists \kappa(o(\kappa) = 1)$ is equiconsistent with
$\aleph_1 \rightarrow (\omega)^{\omega}_{\aleph_0}$.
In [87], we were able to show that the result generalizes to $n = 2$;
namely, $\exists \kappa(o(\kappa) = 2)$ is equiconsistent with
$\aleph_1 \rightarrow (\omega^2)^{\omega^2}_{\aleph_0}$.
Surprisingly at first sight, this property cannot be generalized further (for $n> 2$), and later on we were able to prove that
$\aleph_1 \rightarrow (\omega^3)^{\omega^3}_{\aleph_0}$
is equiconsistent with $\exists \kappa(o(o(\kappa)) = 2)$.
The above lead us to a finer notion of homogeneity:
Definition: Weak Homogeneity is the partition property
$\kappa \xrightarrow{\text{\tiny WH}}(\lambda)^{\eta}_{\mu}$
where the only considered subsequences of $\lambda$
are those that are created by removing (or, complementarily, collecting) only finitely many segments of $\lambda$.
Using week homogeneity we were able to prove the following
for any ordinal $\alpha$ [87]: $\exists \kappa(o(\kappa) = \alpha)$
is equiconsistent with
$\aleph_1 \xrightarrow{\text{\tiny WH}}(\omega^\alpha)^{\omega^\alpha}_{\aleph_0}$.
Later on we were able to characterize the consistency strength of
$\exists \kappa(o(\kappa)~=~\kappa)$, and recently we have arrived at
the main result of this paper:
$\exists \kappa(o(\kappa) = \kappa^+)$ is equiconsistent with
$\aleph_1 \xrightarrow{\text{\tiny WH}}(\aleph_1)^{\aleph_1}_{\aleph_0}$
References:
[78] M. Spector: Natural Sentences of Mathematics which are
independent of $V = L$, $V = L^\mu$ etc., 1978 (preprint).
[87] Y.M. Kimchi: Dissertation, 1987, The Hebrew University of Jerusalem, Israel

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