1,573 research outputs found
What majority decisions are possible
The main result is the following:
Let X be a finite set and D be a non empty family of choice functions for (X
choose 2) closed under permutation of X. Then the following conditions are
equivalent:
(A) for any choice function c on (X choose 2) we can find a finite set J and
c_j in D for j in J such that for any x not= y in X : c{x,y}=y Leftrightarrow
|J|/2<| {j in J:c_j{x,y}= y}| (so equality never occurs)
(B) for some c in D and x in X we have |{y: c{x,y}=y}| not= (|X|-1)/2 . We
then describe what is the closure of a set of choice functions by majority; in
fact, there are just two possibilities (in section 3). In section 4 we discuss
a generalization
Quite Complete Real Closed fields
We prove that any ordered field can be extended to one for which every
decreasing sequence of bounded closed intervals, of any length, has a nonempty
intersection; equivalently, there are no Dedekind cuts with equal cofinality
from both sides. Here we strengthen the results from the published version
Superatomic Boolean Algebras: maximal rigidity
We prove that for any superatomic Boolean Algebra of cardinality >beth_omega
there is an automorphism moving uncountably many atoms. Similarly for larger
cardinals. Any of those results are essentially best possible
Categoricity of an abstract elementary class in two successive cardinals
We investigate categoricity of abstract elementary classes without any
remnants of compactness (like non-definability of well ordering, existence of
E.M. models or existence of large cardinals). We prove (assuming a weak version
of GCH around lambda) that if K is categorical in lambda, lambda^+, LS(K) <=
lambda and 1 <= I(lambda^{++},K)< 2^{lambda^{++}} then K has a model in
lambda^{+++}
Characterizing aleph_epsilon-saturated models of superstable ndop theories by L_{infty, aleph_epsilon}-theory
After the main gap theorem was proved (see [Sh:c]), in discussion, Harrington
expressed a desire for a finer structure - of finitary character (when we have
a structure theorem at all). I point out that the logic L_{infty,aleph_0}(d.q.)
(d.q. stands for dimension quantifier) does not suffice: e.g., for T=Th(lambda
x 2^\omega,E_n)_{n<omega} where (alpha,eta)E_n(beta,nu) =: eta|n=nu|n and for a
subset S of 2^omega we define
M_S = M | {(alpha,eta): [eta in S -> alpha<omega_1] and [eta in 2^\omega
backslash S -> alpha<omega]}.
Hence, it seems to me we should try L_{infty,aleph_epsilon}(d.q.)
(essentially, in C we can quantify over sets which are included in the
algebraic closure of finite sets), and Harrington accepts this interpretation.
Here the conjecture is proved for aleph_epsilon-saturated models. I.e., the
main theorem is M equiv_{L_{infty,aleph_epsilon}(d.q.)} N iff M cong N for
aleph_epsilon--saturated models of a superstable countable (first order) theory
T without dop
Universal in (< lambda)-stable abelian group
A characteristic result is that if 2^{aleph_0}< mu < mu^+< lambda =
cf(lambda)< mu^{aleph_0}, then among the separable reduced p-groups of
cardinality lambda which are (< lambda)-stable there is no universal one
On full Souslin trees
In this note we answer a question of Kunen (15.13 [Mi91]) showing that it is
consistent that there are full Souslin tree
There may be no nowhere dense ultrafilter
We show the consistency of ZFC +''there is no NWD-ultrafilter on omega'',
which means: for every non principle ultrafilter D on the set of natural
numbers, there is a function f from the set of natural numbers to the reals,
such that for some nowhere dense set A of reals, the set {n: f(n) in A} is not
in D. This answers a question of van Douwen, which was put in more general
context by Baumgartne
Large continuum, oracles
Our main theorem is about iterated forcing for making the continuum larger
than aleph_2. We present a generalization of math.LO/0303294 which is dealing
with oracles for random, etc., replacing aleph_1, aleph_2 by lambda,lambda^+
(starting with lambda=lambda^{aleph_1). Well, instead of properness we
demand absolute c.c.c. So we get, e.g. the continuum is lambda^+ but we can get
cov(meagre)=lambda. We give some applications. As in math.LO/0303294, it is a
"partial" countable support iteration but it is c.c.c
Can groupwise density be much bigger than the non-dominating number?
We prove that g (the groupwise density number) is smaller or equal to b^+
(the successor of the minimal cardinality of a non-dominated subset of
omega^omega)
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