238 research outputs found
The bounded proper forcing axiom and well orderings of the reals
We show that the bounded proper forcing axiom BPFA implies that there is a well-ordering of P(Ļ_1) which is Ī_1 definable with parameter a subset of Ļ_1. Our proof shows that if BPFA holds then any inner model of the universe of sets that correctly computes N_2 and also satisfies BPFA must contain all subsets of Ļ_1. We show as applications how to build minimal models of BPFA and that BPFA implies that the decision problem for the HƤrtig quantifier is not lightface projective
The Complexity of Simultaneous Geometric Graph Embedding
Given a collection of planar graphs on the same set of
vertices, the simultaneous geometric embedding (with mapping) problem, or
simply -SGE, is to find a set of points in the plane and a bijection
such that the induced straight-line drawings of
under are all plane.
This problem is polynomial-time equivalent to weak rectilinear realizability
of abstract topological graphs, which Kyn\v{c}l (doi:10.1007/s00454-010-9320-x)
proved to be complete for , the existential theory of the
reals. Hence the problem -SGE is polynomial-time equivalent to several other
problems in computational geometry, such as recognizing intersection graphs of
line segments or finding the rectilinear crossing number of a graph.
We give an elementary reduction from the pseudoline stretchability problem to
-SGE, with the property that both numbers and are linear in the
number of pseudolines. This implies not only the -hardness
result, but also a lower bound on the minimum size of a
grid on which any such simultaneous embedding can be drawn. This bound is
tight. Hence there exists such collections of graphs that can be simultaneously
embedded, but every simultaneous drawing requires an exponential number of bits
per coordinates. The best value that can be extracted from Kyn\v{c}l's proof is
only
Definable MAD families and forcing axioms
We show that under the Bounded Proper Forcing Axiom and an anti-large
cardinal assumption, there is a MAD family.Comment: 13 page
Incompatible bounded category forcing axioms
We introduce bounded category forcing axioms for well-behaved classes
. These are strong forms of bounded forcing axioms which completely
decide the theory of some initial segment of the universe
modulo forcing in , for some cardinal
naturally associated to . These axioms naturally
extend projective absoluteness for arbitrary set-forcing--in this situation
--to classes with .
Unlike projective absoluteness, these higher bounded category forcing axioms do
not follow from large cardinal axioms, but can be forced under mild large
cardinal assumptions on . We also show the existence of many classes
with , and giving rise to pairwise
incompatible theories for .Comment: arXiv admin note: substantial text overlap with arXiv:1805.0873
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