### Dependent dreams: recounting types

by Shelah. [Sh:950]

We investigate the class of models of a general dependent
theory. We continue [Sh:900] in particular investigating
so called ``decomposition of types''; thesis is that
what holds for stable theory and for Th (Q,<) hold for dependent
theories. Another way to say this is: we have to look at small enough
neighborhood and use reasonably definable types to analyze a type. We
note the results understable without reading. First,
a parallel to the ``stability spectrum'', the
``recounting of types'', that is assume lambda = lambda^{< lambda}
is large enough, M a saturated model of T of cardinality
lambda, let bold S_{aut}(M) be the number of complete
types over M up to being conjugate, i.e. we identify p,q when
some automorphism of M maps p to q . Whereas for
independent T the number is 2^lambda, for dependent T
the number is <= lambda moreover it is <=
| alpha |^{|T|} when lambda = aleph_alpha .
Second, for stable theories ``lots of indiscernibility exists''
a ``too good indiscernible existence theorem'' saying,
e.g. that if the type
tp (d_beta ; {d_beta : beta < alpha}) is increasing for alpha <
kappa = text {cf}(kappa) and kappa > 2^{|T|} {then} <
d_alpha : alpha in S> is indiscernible for some stationary S
subseteq kappa . Third, for stable T,a model is
kappa-saturated iff it is aleph_epsilon-saturated and every
infinite indiscernible set (of elements) of cardinality < kappa can
be increased. We prove here an analog. Fourth, for p in bold
S(M), the number of ultrafilters on the outside definable subsets of
M extending p has an absolute bound 2^{|T|} . Restricting
ourselves to one phi (x, bar y), the number is finite, with an
absolute found (well depending on T and phi). Also if M is
saturated then p is the average of an indiscernible sequence inside
the model. Lastly, the so-called generic pair conjecture was proved
in [Sh:900] for kappa measurable, here it is essentially
proved, i.e. for kappa > |T| + beth_omega .

Back to the list of publications