arXiv subjects

Timothy Y. Chow

Publications and source records attributed to Timothy Y. Chow.

At least 19 recordsLinked to original sources

arXiv2026

Foata, Hikita, and the Bulldozer Problem

In a remarkable paper, Tatsuyuki Hikita settled a longstanding e-positivity conjecture of Stanley and Stembridge. Among many other things, he wrote down a certain formula ${\varphi}_k$, and proved that the ${\varphi}_k$ sum to one, thereby defining a probability distribution. Though Hikita's proof was simple, it remains surprising that the ${\varphi}_k$ sum to one. In this note, we give a combinatorial interpretation of Hikita's probability distribution. The main tool is a certain permutation statistic that we call the watershed. After seeing an early version of our work, Darij Grinberg noticed that the permutation statistic was implicit in a so-called "bulldozer problem" that was on the short list for the 2015 International Mathematics Olympiad. However, our description of the statistic, which makes use of the Renyi-Foata bijection, appears to be new.

math.CO↗

arXiv2025

Bluffing in Scrabble

It is well known that in games with imperfect information, such as poker, bluffing with some probability can be a component of the optimal strategy. However, as far as we know, nobody has ever exhibited a Scrabble position in which the optimal strategy involves bluffing, or even a Scrabble position in which the optimal strategy is a mixed (i.e., randomized) strategy. We present a carefully constructed Scrabble position, that could actually arise in a tournament game with no invalid words played, in which the optimal strategy (assuming that a tied score leads to the point being split equally, with no recourse to so-called "spread points" as a tie-breaking mechanism) is to make Move A with probability 1/3 and to make Move B with probability 2/3. Move B can reasonably be called a bluff, in the sense that it sets up a threat which the player cannot in fact execute, but which the opponent may not be able to rule out.

math.HO↗

arXiv2024

The Latin Tableau Conjecture

A Latin tableau of shape $λ$ and type $μ$ is a Young diagram of shape $λ$ in which each box contains a single positive integer, with no repeated integers in any row or column, and the $i$th most common integer appearing $μ_i$ times. Over twenty years ago, Chow et al., in their study of a generalization of Rota's basis conjecture that they called the wide partition conjecture, conjectured a necessary and sufficient condition for the existence of a Latin tableau of shape $λ$ and type $μ$. We report some computational evidence for this conjecture, and prove that the conjecture correctly characterizes, for any given $λ$, at least the first four parts of $μ$.

math.CO↗

arXiv2024

Cooking Poisons: Thinking Laterally with Game Theory

We revive an old lateral-thinking puzzle by Michael Rabin, involving poisons with strange properties. We show that the puzzle admits several unintended solutions that are just as interesting as the intended solution. Analyzing these alternative solutions using game theory yields surprisingly subtle results and several unanswered questions.

math.CO↗

arXiv2024

A well-motivated proof that pi is irrational

Ivan Niven's succinct proof that pi is irrational is easy to verify, but it begins with a magical formula that appears to come out of nowhere, and whose origin remains mysterious even after one goes through the proof. The goal of this expository paper is to describe a thought process by which a mathematician might come up with the proof from scratch, without having to be a genius. Compared to previous expositions of Niven's proof, perhaps the main novelty in the present account is an explicit appeal to the theory of orthogonal polynomials, which leads naturally to the consideration of certain integrals whose relevance is otherwise not immediately obvious.

math.HO↗

arXiv2022

A Mathematician Reads the Kalam Cosmological Argument

Some Christian apologists, notably William Lane Craig, have championed something called the kalam cosmological argument for the existence of God. One version of the argument leans heavily on the claim that the existence of an actual infinite in the physical world is a metaphysical impossibility. We strongly criticize this claim, showing that it involves dogmatically insisting that certain metaphysical premises are absolutely inviolable, when in fact said premises are not only optional, but are far flimsier than other metaphysical claims (eventually shown to be untenable) that great thinkers of the past, including Einstein, have misguidedly clung to. While our criticisms strike most mathematicians and physicists as straightforward and uncontroversial, they have encountered resistance from philosophers, suggesting that there is a communication gap between the scientific and philosophical communities. We hope this paper will help bridge that gap.

math.LO↗

arXiv2020

Algorithmically distinguishing irreducible characters of the symmetric group

Suppose that $χ_λ$ and $χ_μ$ are distinct irreducible characters of the symmetric group $S_n$. We give an algorithm that, in time polynomial in $n$, constructs $π\in S_n$ such that $χ_λ(π)$ is provably different from $χ_μ(π)$. In fact, we show a little more. Suppose $f=χ_λ$ for some irreducible character $χ_λ$ of $S_n$, but we do not know $λ$, and we are given only oracle access to $f$. We give an algorithm that determines $λ$, using a number of queries to $f$ that is polynomial in $n$. Each query can be computed in time polynomial in $n$ by someone who knows $λ$.

math.CO↗

arXiv2018

The Consistency of Arithmetic

In 2010, Vladimir Voevodsky gave a lecture on "What If Current Foundations of Mathematics Are Inconsistent?" Among other things he said that he was seriously suspicious that an inconsistency in PA (first-order Peano arithmetic) might someday be found. About a year later, Edward Nelson announced that he had discovered an inconsistency not just in PA, but in a small fragment of primitive recursive arithmetic. Soon, Daniel Tausk and Terence Tao independently found a fatal error, and Nelson withdrew his claim, stating that consistency of PA was an "open problem." Many mathematicians may find such claims bewildering. Is the consistency of PA really an open problem? If so, would the discovery of an inconsistency in PA cause all of mathematics to come crashing down like a house of cards? This expository article attempts to address these questions, by sketching and discussing existing proofs of the consistency of PA (including Gentzen's proof and Friedman's relative consistency proof that appeals to the Bolzano-Weierstrass theorem). Since Nelson was a self-avowed formalist, the article also examines the implications of formalism.

math.LO↗

arXiv2018

Unit Interval Orders and the Dot Action on the Cohomology of Regular Semisimple Hessenberg Varieties

Motivated by a 1993 conjecture of Stanley and Stembridge, Shareshian and Wachs conjectured that the characteristic map takes the dot action of the symmetric group on the cohomology of a regular semisimple Hessenberg variety to $ωX_G(t)$, where $X_G(t)$ is the chromatic quasisymmetric function of the incomparability graph $G$ of the corresponding natural unit interval order, and $ω$ is the usual involution on symmetric functions. We prove the Shareshian--Wachs conjecture. Our proof uses the local invariant cycle theorem of Beilinson-Bernstein-Deligne to obtain a surjection from the cohomology of a regular Hessenberg variety of Jordan type $λ$ to a space of local invariant cycles; as $λ$ ranges over all partitions, these spaces collectively contain all the information about the dot action on a regular semisimple Hessenberg variety. Using a palindromicity argument, we show that in our case the surjections are actually isomorphisms, thus reducing the Shareshian-Wachs conjecture to computing the cohomology of a regular Hessenberg variety. But this cohomology has already been described combinatorially by Tymoczko; we give a bijective proof (using a generalization of a combinatorial reciprocity theorem of Chow) that Tymoczko's combinatorial description coincides with the combinatorics of the chromatic quasisymmetric function.

math.AG↗

arXiv2011

The surprise examination or unexpected hanging paradox

The apparently trifling unexpected hanging paradox has generated an enormous philosophical literature. We introduce the mathematician to this literature, paying special attention to aspects that involve nontrivial mathematics. This xxx version of the paper contains an exhaustive bibliography that the editors of the Monthly deemed too lengthy to publish. The bibliography will be continually updated and readers are encouraged to inform the author of any omissions that they discover.

math.LO↗

arXiv2009

Almost-natural proofs

Razborov and Rudich have shown that so-called "natural proofs" are not useful for separating P from NP unless hard pseudorandom number generators do not exist. This famous result is widely regarded as a serious barrier to proving strong lower bounds in circuit complexity theory. By definition, a natural combinatorial property satisfies two conditions, constructivity and largeness. Our main result is that if the largeness condition is weakened slightly, then not only does the Razborov-Rudich proof break down, but such "almost-natural" (and useful) properties provably exist. Specifically, under the same pseudorandomness assumption that Razborov and Rudich make, a simple, explicit property that we call "discrimination" suffices to separate P/poly from NP; discrimination is nearly linear-time computable and almost large, having density 2^{-q(n)} where q is a quasi-polynomial function. For those who hope to separate P from NP using random function properties in some sense, discrimination is interesting, because it is constructive, yet may be thought of as a minor alteration of a property of a random function. The proof relies heavily on the self-defeating character of natural proofs. Our proof technique also yields an unconditional result, namely that there exist almost-large and useful properties that are constructive, if we are allowed to call non-uniform low-complexity classes "constructive." We note, though, that this unconditional result can also be proved by a more conventional counting argument. Finally, we give an alternative proof, communicated to us by Salil Vadhan at FOCS 2008, of one of our theorems, and we make some speculative remarks on the future prospects for proving strong circuit lower bounds.

cs.CC↗

arXiv2008

Reduction of Rota's basis conjecture to a problem on three bases

Rota's basis conjecture, open since 1989, states that if B_1, B_2, ..., B_n are n bases of a vector space of rank n, then there is an nxn grid of vectors such that the vectors in the ith row are precisely the elements of B_i and such that every column is also a basis. It is shown that Rota's basis conjecture follows from a similar conjecture that involves only three bases instead of n bases: If M is a matroid of rank n that is a disjoint union of 3 bases, and I_1, ..., I_n are disjoint independent sets with |I_i| <= 3, then there exists an nx3 grid G that contains each element of M exactly once, with the elements of I_i appearing in row i, such that the three columns of G are bases of M.

math.CO↗

arXiv2008

The power of multifolds: Folding the algebraic closure of the rational numbers

It is well known that the usual Huzita-Hatori axioms for origami enable angle trisection but not angle quintisection. Using the concept of a multifold, Lang has achieved quintisection but not arbitrary algebraic numbers. We define the n-parameter multifold and show how to use one-parameter multifolds to obtain the algebraic closure of the rational numbers.

math.AG↗

arXiv2008

A beginner's guide to forcing

This expository paper, aimed at the reader without much background in set theory or logic, gives an overview of Cohen's proof (via forcing) of the independence of the continuum hypothesis. It emphasizes the broad outlines and the intuitive motivation while omitting most of the proofs. The reader must of course consult standard textbooks for the missing details, but this article provides a map of the forest so that the beginner will not get lost while forging through the trees.

math.LO↗

arXiv2008

Simple formulas for lattice paths avoiding certain periodic staircase boundaries

There is a strikingly simple classical formula for the number of lattice paths avoiding the line x = ky when k is a positive integer. We show that the natural generalization of this simple formula continues to hold when the line x = ky is replaced by certain periodic staircase boundaries--but only under special conditions. The simple formula fails in general, and it remains an open question to what extent our results can be further generalized.

math.CO↗

arXiv2004

Fast optical layer mesh protection using pre-cross-connected trails

Conventional optical networks are based on SONET rings, but since rings are known to use bandwidth inefficiently, there has been much research into shared mesh protection, which promises significant bandwidth savings. Unfortunately, most shared mesh protection schemes cannot guarantee that failed traffic will be restored within the 50 ms timeframe that SONET standards specify. A notable exception is the p-cycle scheme of Grover and Stamatelakis. We argue, however, that p-cycles have certain limitations, e.g., there is no easy way to adapt p-cycles to a path-based protection scheme, and p-cycles seem more suited to static traffic than to dynamic traffic. In this paper we show that the key to fast restoration times is not a ring-like topology per se, but rather the ability to pre-cross-connect protection paths. This leads to the concept of a pre-cross-connected trail or PXT, which is a structure that is more flexible than rings and that adapts readily to both path-based and link-based schemes and to both static and dynamic traffic. The PXT protection scheme achieves fast restoration speeds, and our simulations, which have been carefully chosen using ideas from experimental design theory, show that the bandwidth efficiency of the PXT protection scheme is comparable to that of conventional shared mesh protection schemes.

cs.NI↗

arXiv2003

The ring grooming problem

The problem of minimizing the number of bidirectional SONET rings required to support a given traffic demand has been studied by several researchers. Here we study the related ``ring grooming problem'' of minimizing the number of add/drop locations instead of the number of rings; in a number of situations this is a better approximation to the true equipment cost. Our main result is a new lower bound for the case of uniform traffic. This allows us to prove that a certain simple algorithm for uniform traffic is in fact a constant-factor approximation algorithm, and it also demonstrates that known lower bounds for the general problem--in particular, the linear programming relaxation--are not within a constant factor of the optimum. We also show that our results for uniform traffic extend readily to the more practically important case of quasi-uniform traffic. Finally, we show that if the number of nodes on the ring is fixed, then ring grooming is solvable in polynomial time; however, whether ring grooming is fixed-parameter tractable is still an open question.

math.OC↗

arXiv2002

Wide partitions, Latin tableaux, and Rota's basis conjecture

Say that mu is a ``subpartition'' of an integer partition lambda if the multiset of parts of mu is a submultiset of the parts of lambda, and define an integer partition lambda to be ``wide'' if for every subpartition mu of lambda, mu >= mu' in dominance order (where mu' denotes the conjugate or transpose of mu). Then Brian Taylor and the first author have conjectured that an integer partition lambda is wide if and only if there exists a tableau of shape lambda such that (1) for all i, the entries in the ith row of the tableau are precisely the integers from 1 to lambda_i inclusive, and (2) for all j, the entries in the jth column of the tableau are pairwise distinct. This conjecture was originally motivated by Rota's basis conjecture and, if true, yields a new class of integer multiflow problems that satisfy max-flow min-cut and integrality. Wide partitions also yield a class of graphs that satisfy ``delta-conjugacy'' (in the sense of Greene and Kleitman), and the above conjecture implies that these graphs furthermore have a completely saturated stable set partition. We present several partial results, but the conjecture remains very much open.

math.CO↗