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August 2023
Elena Fuchs' work discussed in Quanta magazine
What sort of collections of circles fit into a larger circle without overlapping? This question has motivated mathematicians for millenia. Here at UC Davis, Elena Fuchs has studied Apollonian circle packings and found that the curvatures appearing in such families of circles exhibit certain combinatorial features. Her results have led to stronger conjectures, widely accepted among number theorists, but one of these conjectures turned out to be false. This was discovered (via extensive number crunching) during a summer research project involving two students (one graduate, one undergraduate) led by Katherine Stange at the University of Colorado in Boulder. An article in Quanta magazine tells the story of the students' discovery and provides mathematical details.
August 2023
Two successful conferences wrapped up in Summer '23

This July, our Department hosted two conferences! Both were part of annual traditions, namely the Trisectors workshops and the Formal Power Series and Algebraic Combinatorics (FPSAC) annual meetings.

The Trisectors Workshop took place June 26-June 30 and was organized by Alex Zupan, Laura Starkston, Jeffrey Meier, Maggie Miller, and Gabe Islambouli. This year's workshop emphasized connections with symplectic topology. It was preceded by introductory lectures delivered over Zoom during the week leading up to the conference and featured several afternoon devoted to group projects.

The FPSAC meeting, a much larger event, took place July 17-21 and was organized by a much longer list of people including several from UC Davis: Monica Vazirani, Matt Silver, Anne Schilling, Dan Romik, Alex McDonough, Gladis Lopez, Fu Liu, Shelby Kustak, Sean Griffin, Tina Denena, Jesus DeLoera, and Eric Carlsson.

Both conferences were reported to be very successful! Let's thank our colleagues and staff for their hard work!

Learn more about these conferences at the websites for FPSAC 2023 and Trisectors Workshop 2023.

June 2023
Professor Casals selected as College of Letters and Science Dean's Faculty Fellow
UC Davis' Letters and Science has selected Professor Roger Casals as a College of Letters and Science Dean's Faculty Fellow for 2023.
June 2023
Professor Chaudhuri's research on decision-making featured
UC Davis' Letters and Science interviewed Professor Rishidev Chaudhuri about the details of his research, which spans neuroscience and math. The article, Perception Inception: Exploring Decision-Making in the Brain with Rishidev Chaudhuri, provides an accessible view into what he's researching, and why he finds it interesting.
November 2022
Prof. Babson on Nontransitive Dice

Moon Duchin and Dylan Thurston (erstwhile KAP and UCD faculty child respectively) brought the following question arising from gerrymandering:

Identify the subset $P_n$ of the simplex $\Delta$ in $\mathbb{R}^{n!}$ (viewed as all probability measures on permutations) obtained by fixing $n$ (typically different) probability measures on $\mathbb{R}$, sampling one point from each and recording the resulting permutation.

For example the midpoint or corners of $\Delta$ result if the measures are equal or each have distinct one point support respectively.

On the other hand $P_n$ is not all of $\Delta$. In particular a global quadratic inequality follows from the correlation between the conditions $a<b$ and $a<c$. Already for $n=3$ if $[abc]$ is the probability that $a<b<c$ then $[abc][cba]\leq([acb]+[cab])([bac]+[bca])$ so for instance the midpoint of the edge in $\Delta$ between $[abc]$ and $[cba]$ is not in $P_3$. Many such global quadratics have been written down by Fontain, Kasteleyn and Ginibre.

This ostensibly measure related problem becomes algebra upon noting that it suffices to consider measures of finite support and that for each fixed collection $\sigma=(\sigma_i)|_{i\in[n]}$ of disjoint finite support sets the associated region $P_\sigma\subseteq P_n$ is the image of the positive real points of a rational variety in $\mathbb{P}^{n!-1}$. For example with $n=3$ and $\sigma=(\{1,5\},\{2,4\},\{3\})$ there is a square of possible measures with these supports indexed by the probability $x$ for $1$ (rather than $5$) in the first measure and $y$ for $2$ in the second. The image $P_\sigma$ of this square in $\Delta$ is the ruled surface given by $[bac]=[cab]=0$ and $[abc][cab]=[acb][bca]$ and is the positive part of the Segre embedding of $\mathbb{P}^1\times\mathbb{P}^1$ into $\mathbb{P}^3$. More commonly the initial product of projective spaces will require some resolution away from the positive part before the rational map becomes algebraic.

A second connection to algebra is that when viewing $\sigma$ as a sequence of numbers from $[n]$ so that the example above becomes $abcba$ a braid move such as to $acbca$ does not change $P_\sigma$ though it does change the associated coordinatization. Thus it suffices to consider $\sigma$ indexed by elements of the $K_n$ Coxeter group. For $n=2$ this is affine type A and there are up to the action of $S_3$ only $\lceil\frac{k}{2}\rceil$ words of length $k$ required to get all of the sets $P_\sigma$ so for $k=5$ there are only $abcba$, $abcac$ and $abcab$.

Even for three measures there is more to the story. $P_3$ is full (five) dimensional in $\Delta$ and covered by the four $S_3$ orbits of $\{P_\sigma\}$ with $\sigma$ of length eight. The boundaries of these $P_\sigma$ besides the linear positivity requirements and the above FKG quadratics are defined by two other $S_3$ orbits of divisors (of degrees three and four) which we only found by computer and which do not give global inequalities for $P_3$. The analogs for higher $n$ remain quite mysterious.

The case of three measures is also essentially Efron's nontransitive or ro-cham-bo dice problem: Find three weighted dice (rock, paper and scissors) with various face values (these are the three measures) for which the probabilities that paper beats rock, scissors beats paper and rock beats scissors are all more than half. For example if you ask that all three of these probabilities agree and be as large as possible this will be a boundary point of $P_3$ along a diagonal line and contained in $P_{abcabca}$ which is a four dimensional divisor satisfying the quartic. (Since there are only two b's and c's the paper and scissors dice can be replaced with coins.)

April 2022
Strohmer awarded $1.2 million for data science in health
Together with two UC Davis Health PIs--Racheael Callcut, professor of surgery and chief research informatics officer, and Jason Adams, associate professor and physician of pulmonary, critical care and sleep--Thomas Strohmer was awarded a $1.2 million grant from the National Institutes of Health (NIH). The grant aims to fund the generation of high-quality synthetic data using artificial intelligence and machine learning (AI/ML) to potentially help physicians predict, diagnose and treat diseases. The Center for Data Science and Artificial Intelligence (CEDAR) at UC Davis, launched in 2019 by Thomas Strohmer, promotes interdisciplinary research. The results are beginning to show. Congratulations Thomas!