arXiv subjects

James Hirschorn

Publications and source records attributed to James Hirschorn.

10 recordsLinked to original sources

arXiv2023

A Measure Theoretic Proof of $\mathfrak p=\mathfrak t$

Rothberger's question of whether the two cardinals $\mathfrak p$ and $\mathfrak t$ are equal, posed back in 1948, was only answered fairly recently in the affirmative. Here we answer the more difficult progenitor question (posed in the same place) on when filter-bases can be refined to towers. Of equal import, our proof that $\mathfrak p=\mathfrak t$ addresses issues raised by Gowers concerning the "proof path" of the original solution. This is applied to obtain a gap spectrum result for the ultrapowers $\mathbb N^{\mathbb N}\,/\,{\mathcal U}$.

math.LO↗

arXiv2008

Nonhomogeneous analytic families of trees

We consider a dichotomy for analytic families of trees stating that either there is a colouring of the nodes for which all but finitely many levels of every tree are nonhomogeneous, or else the family contains an uncountable antichain. This dichotomy implies that every nontrivial Souslin poset satisfying the countable chain condition adds a splitting real. We then reduce the dichotomy to a conjecture of Sperner Theory. This conjecture is concerning the asymptotic behaviour of the product of the sizes of the m-shades of pairs of cross-t-intersecting families.

math.LO↗

arXiv2008

Asymptotic upper bounds on the shades of t-intersecting families

We examine the m-shades of t-intersecting families of k-subsets of [n], and conjecture on the optimal upper bound on their cardinalities. This conjecture extends Frankl's General Conjecture that was proven true by Ahlswede-Khachatrian. From this we deduce the precise asymptotic upper bounds on the cardinalities of m-shades of t(m)-intersecting families of k(m)-subsets of [2m], as m -> infinity. A generalization to cross-t-intersecting families is also considered.

math.CO↗

arXiv2008

On the strength of Hausdorff's gap condition

Hausdorff's gap condition was satisfied by his original 1936 construction of an (omega-1,omega-1) gap in P(N)/Fin. We solve an open problem in determining whether Hausdorff's condition is actually stronger than the more modern indestructibility condition, by constructing an indestructible (omega-1,omega-1) gap not equivalent to any gap satisfying Hausdorff's condition, from uncountably many random reals.

math.LO↗

arXiv2008

A strong antidiamond principle compatible with CH

A strong antidiamond principle (*c) is shown to be consistent with CH. This principle can be stated as a "P-ideal dichotomy": every P-ideal on omega-1 (i.e. an ideal that is sigma-directed under inclusion modulo finite) either has a closed unbounded subset of omega-1 locally inside of it, or else has a stationary subset of omega-1 orthogonal to it. We rely on Shelah's theory of parameterized properness for NNR iterations, and make a contribution to the theory with a method of constructing the properness parameter simultaneously with the iteration. Our handling of the application of the NNR iteration theory involves definability of forcing notions in third order arithmetic, analogous to Souslin forcing in second order arithmetic.

math.LO↗

arXiv2008

Combinatorial and hybrid principles for sigma-directed families of countable sets modulo finite

We consider strong combinatorial principles for sigma-directed families of countable sets in the ordering by inclusion modulo finite, e.g. P-ideals of countable sets. We try for principles as strong as possible while remaining compatible with CH, and we also consider principles compatible with the existence of nonspecial Aronszajn trees. The main thrust is towards abstract principles with game theoretic formulations. Some of these principles are purely combinatorial, while the ultimate principles are primarily combinatorial but also have aspects of forcing axioms.

math.LO↗

arXiv2007

Random gaps

It is proved that there exists an (omega-1,omega-1) Souslin gap in the Boolean algebra (L(nu)/Fin,subseteq^*_ae) for every nonseparable measure nu. Thus a Souslin, also known as destructible, (omega-1,omega-1) gap in P(N)/Fin can always be constructed from uncountably many random reals. We explain how to obtain the corresponding conclusion from the hypothesis that Lebesgue measure can be extended to all subsets of the real line (RVM).

math.LO↗

arXiv2007

Partial order embeddings with convex range

A careful study is made of embeddings of posets which have a convex range. We observe that such embeddings share nice properties with the homomorphisms of more restrictive categories; for example, we show that every order embedding between two lattices with convex range is a continuous lattice homomorphism. A number of posets are considered; for example, we prove that every product order embedding sigma between the irrationals (i.e. the family of functions from N into N) with convex range is of the form sigma(x)(n) = ((x o g) + y)(n) if n in K, and sigma(x)(n) = y(n) otherwise, for all irrationals x, where K is a subset of N, g:K -> N is a bijection and y is an irrational. The most complex poset examined here is the quotient of the lattice of Baire measurable functions, with codomain of the form N^I for some index set I, modulo equality on a comeager subset of the domain, with its `natural' ordering.

math.RA↗

arXiv2006

Pinning quasi orders with their endomorphisms

Some general properties of abstract relations are closely examined. These include generalizations of linearity, and properties based on `pinning' an inequality by a pair of families of endomorphisms.To each property we try to associate a canonical definition of an augmentation (or diminishment) that augments (or diminishes) any given relation to one satisfying the desired property. The motivation behind this study was to identify properties distinguishing between the product ordering and the eventual dominance ordering of the irrationals (the family of functions from N into N), and furthermore to identify their relationship as a member of a natural class of augmentations.

math.RA↗

arXiv2005

Some partition properties for measurable colourings of omega-one^2

We construct a measure on omega-one^2 over the ground model in the forcing extension of a measure algebra, and investigate when measure theoretic properties of some measurable colouring of omega-one^2 imply the existence of an uncountable subset of omega-one whose square is homogeneous. This gives a new proof of the fact that, under a suitable axiomatic assumption, there are no Souslin (omega-one,omega-one) gaps in the Boolean algebra L^0(nu)/Fin when nu is a separable measure.

math.LO↗