Henry Adams

Talks

Upcoming talks, mini-symposia, and conferences

~~Giving a two-day workshop on applied topology at The 54th Spring Topology and Dynamical Systems Conference, Murray State University, KY, Mar 18-21, 2020.~~
~~Speaking on Descriptors of Energy Landscapes using Topological Analysis (DELTA) at the Special Session on Mathematics of Data Science, AMS Sectional Meeting, Tufts University, MA, Mar 21-22, 2020.~~ [Abstract]

Many of the properties of a chemical system are described by its energy landscape, a real-valued function defined on a high-dimensional domain. I will explain how topology, and in particular persistent homology, can be used in order to describe some of the pertinent features of an energy landscape. As a motivating example, the low-energy conformation space of an aluminate molecular anion interacting with a simple potassium cation can be well-modeled by the permutohedron (joint work with Aurora Clark and Hung Le). In a recently funded NSF Harnessing the Data Revolution project, the DELTA team is learning how to identify and leverage changing topological features of energy landscapes across a range of chemical conditions in order to predict reactivity.
Speaking on Computational topology and energy landscapes at the National Meeting of the American Chemical Society, for the Graph Theory Underpinning New Domains of Physical Chemistry symposium, Philadelphia, PA, Mar 22-26, 2020. [Abstract]

This talk is an introduction to applied and computational topology, emphasizing applications to datasets arising in the study of chemical energy landscapes. One such dataset is the conformation space of the cyclo-octane molecule, which is a Klein bottle glued to a 2-sphere along two circles. I will introduce persistent homology as a topological tool for visualizing, understanding, and performing machine learning tasks on high-dimensional datasets.
~~Speaking at the University of Pennsylvania's Applied Topology Seminar, Mar 24, 2020.~~
Speaking on Applying for academic jobs at the Student Chapter of the AMS at CSU, Apr 14, 2020.
~~Speaking at Topology day, CU Boulder, CO, Apr 21, 2020.~~ (Topology day is cancelled, but perhaps will be rescheduled.)
Attending an NSF Harnessing the Data Revolution Coordination Workshop & PI Meeting in Alexandria, VA, April 28-30, 2020. (Moved to an online meeting.)
Speaking on From persistent homology output to machine learning input at the SIAM Conference on Mathematics of Data Science (MDS20), for the Minisymposium on Topological Image Analysis, Cincinnati, OH, May 5-7, 2020. (This event will perhaps be rescheduled for 2021.) [Abstract]

I will give an overview of a variety of ways to turn persistent homology output into input for machine learning tasks, including a discussion of the stability and interpretability properties of these methods. Persistent homology is a reasonable summary not only of the global topology but also of the local geometry present in a dataset. When persistent homology is interpreted by humans, the perspective is that the most persistent features matter the most, but machine learning algorithms heavily use low-persistent features measuring local geometry.
Speaking on Borsuk-Ulam theorems into higher-dimensional codomains at a mini-symposium on Geometric and Topological Combinatorics and Applications at the SIAM Conference on Discrete Mathematics, Portland, OR, June 1-4, 2020. (This event will potentially be rescheduled.) [Abstract]
Speaking on Borsuk-Ulam theorems into higher-dimensional codomains at the Workshop on Computational Geometry and Topology at Foundations of Computational Mathematics (FoCM), Vancouver, Canada, June 15-17, 2020. (This event will perhaps be rescheduled for 2021.) [Abstract]

I will describe Borsuk-Ulam theorems into higher-dimensional codomains. One formulation of the Borsuk-Ulam theorem is that any odd map from the n-dimensional sphere into Euclidean space Rⁿ hits the origin. We generalize this result by increasing the dimension of the codomain, and by finding a small-diameter subset of the sphere whose image contains the origin in its convex hull. For example, given an odd map from the circle of circumference one into R^2k+1, there is some subset of the circle of diameter at most k/(2k+1) whose image contains the origin in its convex hull, and this diameter bound is sharp. We will pose open questions for the n-sphere, where optimal diameters are known only for odd maps of the n-sphere into Rⁿ⁺¹ and Rⁿ⁺². Furthermore, we will explain how these Borsuk-Ulam theorems are related to Vietoris-Rips complexes of spheres.
Joint work with Johnathan Bush and Florian Frick.
Speaking on Metric reconstruction via optimal transport at the Workshop on Optimal Transport, Topological Data Analysis, and Applications to Shape and Machine Learning, Mathematical Biosciences Institute - The Ohio State University, July 28-31, 2020. [Abstract]

Given a sample of points X in a metric space M and a scale r > 0, the Vietoris-Rips simplicial complex VR(X;r) is a standard construction to attempt to recover M from X up to homotopy type. A deficiency is that VR(X;r) is not metrizable if it is not locally finite, and thus does not recover metric information about M. We remedy this shortcoming by defining the Vietoris-Rips metric thickening VR^m(X;r) via the theory of optimal transport. When M is a Riemannian manifold, the Vietoris-Rips thickening satisfies Hausmann's theorem (VR^m(M;r) is homotopy equivalent to M for r sufficiently small) with a simpler proof: homotopy equivalence VR^m(M;r) → M is now canonically defined as a center of mass map, and its homotopy inverse is the (now continuous) inclusion map M → VR^m(M;r). We discuss how Vietoris-Rips thickenings of circles and n-spheres are related to Borsuk-Ulam theorems into higher-dimensional codomains.
Joint work with Michał Adamaszek, John Bush, and Florian Frick.
Speaking on Metric reconstruction and Borsuk-Ulam theorems at the Tulane University Geometry and Topology Seminar, New Orleans, LA, Aug 27, 2020. [Abstract]

Given a sample of points X in a metric space M and a scale r > 0, the Vietoris-Rips complex is a standard construction to attempt to recover M from X up to homotopy type. A deficiency is that the Vietoris-Rips simplicial complex may not be metrizable if X is infinite, and therefore need not recover metric information about M. We remedy this shortcoming by defining the Vietoris-Rips metric thickening via the theory of optimal transport. With this machinery, we can say more about the homotopy type of Vietoris-Rips thickenings of n-spheres. As an application, we give generalizations of the Borsuk-Ulam theorem for maps from the n-sphere into higher-dimensional codomains R^k with k ≥ n.
Giving a talk at the Special Session on Topological Data Analysis and Artificial Intelligence, AMS Fall Southeastern Sectional Meeting, University of Tennessee at Chattanooga, TN, Oct 10-11, 2020.
Attending the Workshop on Computational Topology and Applications, Tsinghua Sanya International Mathematics Forum (TSIMF) in Sanya, China, Dec 7-11, 2020.

Talks for undergraduate students

It's important that mathematicians target some of our talks at a level appropriate for undergraduate audiences. This is perhaps especially true in applied topology, since it is an attractive area for introducing students to research. The slides for two talks that I like to give for undergraduates, in which I try to focus on pictures and de-emphasize formulas, are available here and here.

Research talks

Gave a mini-course on Geometric complexes in applied topology at the Winter School on Geometric and Topological Data Analysis at CIMAT, in Guanajuato, Mexico, Jan 2020. [Abstract, Notes]

The course will begin with an introduction to applied an computational topology. The shape of a dataset often reflects important patterns within. One such dataset with an interesting shape is the conformation space of the cyclo-octane molecule, which is a Klein bottle glued to a 2-sphere along two circles. I will introduce persistent homology as a topological tool for learning properties of a space from only a finite sample, and hence for visualizing, understanding, and performing machine learning tasks on high-dimensional datasets.
Equipped with this motivation, we will move to a mathematical study of geometric complexes arising in applied topology, including nerve, Cech, alpha, and Vietoris-Rips complexes. These complexes thicken a finite sampling into a simplicial complex approximating the underlying space. We will begin by studying these complexes with first principles, but will advertise many open questions and current research directions throughout.
Vietoris-Rips thickenings of spheres. AMS Special Session on Vietoris-Borsuk-Rips Homotopy, JMM, Denver, Jan 2020. [Abstract, Slides]

If a dataset is sampled from a manifold, then as more and more samples are drawn, the persistent homology of the Vietoris-Rips complex of the dataset converges to the persistent homology of the Vietoris-Rips complex of the manifold. But very little is known about the homotopy types of Vietoris-Rips complexes of manifolds. An exception is the case of the circle: as the scale parameter increases, the Vietoris-Rips complex of the circle obtains the homotopy types of the circle, the 3-sphere, the 5-sphere, the 7-sphere, ..., until finally it is contractible. The Vietoris-Rips complex of the n-sphere first obtains the homotopy type of the n-sphere, and then next the (n+1)-fold suspension of a (topological) quotient of the special orthogonal group SO(n+1) by an alternating group A_n+2. Nothing is known at later scales, although one could conjecture that the critical scale parameters of Vietoris-Rips complexes of spheres are related to Lovász' strongly self-dual polytopes. This is joint work with Michał Adamaszek and Florian Frick.
Metric reconstruction via optimal transport. [Abstract, Slides, Notes]

Given a metric space X and a distance threshold r > 0, the Vietoris-Rips simplicial complex has as its simplices the finite subsets of X of diameter less than r. A theorem of Jean-Claude Hausmann states that if X is a Riemannian manifold and r is sufficiently small, then the Vietoris-Rips complex is homotopy equivalent to the original manifold. Janko Latschev proves an analogous theorem for sufficiently dense samplings. Little is known about the behavior of Vietoris-Rips complexes for larger values of r, even though these complexes arise naturally when using persistent homology. We describe how as r increases, the Vietoris-Rips complex of the circle obtains the homotopy types of the circle, the 3-sphere, the 5-sphere, the 7-sphere, ..., until finally it is contractible. These homotopy types are related to cyclic and centrally symmetric polytopes and orbitopes. We argue that infinite Vietoris-Rips complexes should be equipped with a different topology: an optimal transport or Wasserstein metric thickening the metric on X. With this new metric, we describe the first change in homotopy type (as r increases) of Vietoris-Rips complexes of higher-dimensional spheres. Joint work with Michał Adamaszek and Florian Frick.
- Arches Topology Conference, Hurricane, Utah, Apr 2019.
- AMS Special Session on Topological Data Analysis: Theory and Applications, Joint Mathematics Meetings, Baltimore, Jan 2019. [Slides with an application to Borsuk-Ulam theorems]
- Topology Seminar, Texas State University, Nov 2018. [Abstract]
- Upstate New York Topology Seminar (UNYTS), University of Albany, Nov 2018. [Notes]
- Lafayette-Lehigh Geometry-Topology Seminar, Apr 2018. [Notes]
- AATRN Online Seminar, Oct 2017. [Video]
- 58th Cascade Topology Seminar, University of British Columbia, May 2017.
- Florida International University Winter Conference on Geometry, Topology, and Applications, Jan 2017. [Abstract, Old Notes]
An introduction to applied topology. [Abstract]

This talk is an introduction to computational topology, as applied to data analysis and to sensor networks. The shape of a dataset often reflects important patterns within. Two such datasets with interesting shapes are a space of 3x3 pixel patches from optical images, which can be well-modeled by a Klein bottle, and the conformation space of the cyclo-octane molecule, which is a Klein bottle glued to a 2-sphere along two circles. I will introduce topological tools (persistent homology) for visualizing, understanding, and performing machine learning tasks on high-dimensional datasets. As a second application, I will describe how topology has been applied to coverage problems in mobile sensor networks.
- Applied Mathematics Colloquium, CU Boulder, Sep 2019. [Abstract, Slides]
- Three talk lecture series, along with a software tutorial on persistent homology, at the Summer School on Data Science for Dynamical Systems, Lorentz Center, Leiden University, Netherlands, July 2019. [Slides]
- The Symposium of Physics and Mathematics FCFM-IFM at the University of Michoacan, Morelia, Mexico, Nov 2018. [Abstract, Slides]
- Department Colloquium, Texas State University, Nov 2018. [Abstract, Slides]
- Data Science Seminar, University of Tennessee, Nov 2018. [Slides]
- Undergraduate colloquium at Lafayette College, Apr 2018. [Abstract, Slides]
- Florida International University Winter Conference on Geometry, Topology, and Applications, Jan 2017. [Abstract, Slides]
- CSU Computer Science Colloquium, Oct 2016. [Abstract, Slides, Video]
An introduction to applied topology software. NSF-CBMS Conference and Software Day on Topological Methods in Machine Learning and Artificial Intelligence, College of Charleston, South Carolina, May 2019. Also led a week-long research conversation group, and an afternoon-long coding sprint on real-life applied topology examples for beginners. [Abstract, Slides, Video]

I will survey of a wide variety of software packages for applied and computational topology. We learn software best by running it on examples, and I will describe some of my favorite exercises for students, including sampling points from a torus, Klein bottle, or projective plane. Two real-world datasets that can be analyzed with applied topology software include the three-circle model for optical image patches, and the conformation space of the cyclo-octane molecule (which is a Klein bottle glued to a sphere along two circles of singularities). I'll conclude by describing two applied topology online research seminars and YouTube channels, which allow one to remain connected to the community.
Survey: From persistent homology to machine learning feature vectors. TRIPODS Summer Bootcamp on Topology and Machine Learning, ICERM, Aug 2018. [Abstract, Slides]

I will give an overview of a variety of ways to turn persistent homology output into input for machine learning tasks, including their stability and interpretability properties. Persistent homology is a reasonable summary not only of the global topology but also of the local geometry present in a dataset. When persistent homology is interpreted by humans, the perspective is that the most persistent features matter the most, but machine learning algorithms heavily use low-persistent features measuring local geometry.
Connections between Čech complexes and Carathéodory orbitopes. Special Session on Algebraic, Geometric, and Topological Methods in Combinatorics at the AMS Sectional Meeting, Northeastern University, Boston, Apr 2018. [Slides]
The theory of Vietoris-Rips complexes.
- AMS special session on Topological Data Analysis, Joint Mathematics Meetings in San Diego, Jan 2018. [Abstract, Slides]
- Applied Topology Seminar, Brown University, March 2017. [Abstract, Notes]
  
  Given a metric space X and a distance threshold r > 0, the Vietoris-Rips simplicial complex has as its simplices the finite subsets of X of diameter less than r. A theorem of Jean-Claude Hausmann states that if X is a Riemannian manifold and r is sufficiently small, then the Vietoris-Rips complex is homotopy equivalent to the original manifold. Janko Latschev proves an analogous theorem for sufficiently dense samplings. Little is known about the behavior of Vietoris-Rips complexes for larger values of r, even though these complexes arise naturally in applications of using persistent homology. We describe how as r increases, the Vietoris-Rips complex of the circle obtains the homotopy types of the circle, the 3-sphere, the 5-sphere, the 7-sphere, ..., until finally it is contractible. These homotopy types are related to cyclic and centrally symmetric polytopes and orbitopes. Interestingly, certain Vietoris-Rips complexes of ellipses contain ephemeral summands in their persistent homology modules. We argue that infinite Vietoris-Rips complexes should be equipped with a different topology: an optimal transport or Wasserstein metric thickening the metric on X. With this new metric, we describe the first change in homotopy type (as r increases) of Vietoris-Rips complexes of higher-dimensional spheres. Joint work with Michał Adamaszek, Florian Frick, and Samadwara Reddy.
- Mini-symposium on Applied and Computational Topology at the SIAM Central States Section Meeting, Oct 2016. [Slides]
Vietoris-Rips complexes of circles, ellipses, and higher-dimensional spheres.
- Topology, Geometry, and Data Analysis seminar at The Ohio State University, Feb 2017. [Abstract, Notes]
  
  Given a metric space X and a distance threshold r > 0, the Vietoris-Rips simplicial complex has as its simplices the finite subsets of X of diameter less than r. A theorem of Jean-Claude Hausmann states that if X is a Riemannian manifold and r is sufficiently small, then the Vietoris-Rips complex is homotopy equivalent to the original manifold. Janko Latschev proves an analogous theorem for sufficiently dense samplings. Little is known about the behavior of Vietoris-Rips complexes for larger values of r, even though these complexes arise naturally in applied topology and persistent homology. We describe how as r increases, the Vietoris-Rips complex of the circle obtains the homotopy types of the circle, the 3-sphere, the 5-sphere, the 7-sphere, ..., until finally it is contractible. These homotopy types are connected to cyclic and centrally symmetric polytopes and orbitopes. Interestingly, Latschev's result fails for the ellipse with larger r values, and certain Vietoris-Rips complexes of ellipses contain ephemeral summands in their persistent homology modules. We argue that infinite Vietoris-Rips complexes should be equipped with a different topology: an optimal transport or Wasserstein metric thickening the metric on X. Using this new metric, we describe the first change in homotopy type (as r increases) of Vietoris-Rips complexes of higher-dimensional spheres. Joint work with Michał Adamaszek, Florian Frick, and Samadwara Reddy.
- Joint Mathematics Meetings, AMS Special Session on Applied and Computational Topology, Jan 2016. [Abstract, Slides]
Cyclic polytopes and nerve complexes. Rocky Mountain Algebraic Combinatorics Seminar, Oct 2016. [Abstract, Notes]
What is topology, and how is it applied to data analysis? Front Range Computational & Systems Biology Symposium, July 2016.
Random cyclic dynamical systems. Rocky Mountain Algebraic Combinatorics Seminar, Sept 2015. [Abstract]
The Vietoris-Rips complexes of a circle. [Abstract, Slides, Poster]

Given a metric space X and a distance threshold r > 0, the Vietoris-Rips simplicial complex has as its simplices the finite subsets of X of diameter less than r. A theorem of Jean-Claude Hausmann states that if X is a Riemannian manifold and r is sufficiently small, then the Vietoris-Rips complex is homotopy equivalent to the original manifold. Little is known about the behavior of Vietoris-Rips complexes for larger values of r, even though these complexes arise naturally in applications using persistent homology. We show that as r increases, the Vietoris-Rips complex of the circle obtains the homotopy types of the circle, the 3-sphere, the 5-sphere, the 7-sphere, ..., until finally it is contractible. As our main tool we introduce a directed graph invariant, the winding fraction, which in some sense is dual to the circular chromatic number. Using the winding fraction we classify the homotopy types of the Vietoris-Rips complex of an arbitrary (possibly infinite) subset of the circle, and we study the expected homotopy type of the Vietoris-Rips complex of a uniformly random sample from the circle. Moreover, we show that as the distance parameter increases, the ambient Čech complex of the circle also obtains the homotopy types of the circle, the 3-sphere, the 5-sphere, the 7-sphere, ..., until finally it is contractible. Joint with Michał Adamaszek.
- University of Rochester Data Science Colloquium, Apr 2015.
- Applied Algebraic Topology Research Network, Online Seminar Series, Mar 2015. [Video]
- Colloquium at Colorado State University, Jan 2015.
- Geometry and Topology Seminar at Tulane University, Nov 2014.
- Applied Topology Seminar at the University of Pennsylvania, Nov 2014.
- Graduate and Faculty Seminar at Duke University, Oct 2014.
- Colloquium at UNC Greensboro, Oct 2014.
- Geometry and Topology Seminar at North Carolina State University, Sept 2014.
- IMA Postdoc Seminar, May 2014.
- IMA Postdoc Seminar, Oct 2013.
Evasion paths in mobile sensor networks. [Abstract, Slides, SlidesForUndergrads, Poster, Multimedia]

Suppose that ball-shaped sensors wander in a bounded domain. A sensor doesn't know its location but does know when it overlaps a nearby sensor. We say that an evasion path exists in this sensor network if a moving intruder can avoid detection. In "Coordinate-free coverage in sensor networks with controlled boundaries via homology", Vin de Silva and Robert Ghrist give a necessary condition, depending only on the time-varying connectivity data of the sensors, for an evasion path to exist. Using zigzag persistent homology, we provide an equivalent condition that moreover can be computed in a streaming fashion. However, no method with time-varying connectivity data as input can give necessary and sufficient conditions for the existence of an evasion path. Indeed, we show that the existence of an evasion path depends not only on the fibrewise homotopy type of the region covered by sensors but also on its embedding in spacetime. For planar sensors that also measure weak rotation and distance information, we provide necessary and sufficient conditions for the existence of an evasion path. Joint with Gunnar Carlsson.
- Williams College Colloquium, Mar 2018.
- Topological data analysis of exclusion zones, Edinburgh, Scotland, Dec 2017.
- CSU Pattern Analysis Lab, Oct 2015.
- Duke Graduate & Faculty Seminar, Feb 2015.
- IMA Workshop on Topological Systems: Communication, Sensing, and Actuation, Mar 2014. [Video]
- Rocky Mountain Algebraic Combinatorics Seminar, Nov 2013.
- SIAM Conference on Applied Algebraic Geometry, Aug 2013.
- Applied Topology in Będlewo, July 2013.
- MSRI Workshop on Algebraic Topology, June 2013. [Video]
- Ayasdi Topology Day, June 2013.
- Stanford CompTop Seminar, May 2013.
- Special Session on Applied and Computational Topology at MAA MathFest, Aug 2012.
- Algebraic Topology: Applications and New Directions, July 2012.
- Minisymposium on Applied Algebraic Topology at SIAM Annual Meetings, July 2012. [Video]
- Schloss Dagstuhl Seminar on Applications of Combinatorial Topology to Computer Science, Mar 2012.
- AMS Special Session on Computational and Applied Topology, Joint Meetings, Jan 2012.
- SIAM Conference on Applied Algebraic Geometry, Oct 2011.
Nudged elastic band in topological data analysis. [Abstract]

We use the nudged elastic band method from computational chemistry to analyze high-dimensional data. Our approach is inspired by Morse theory, and as output we produce an increasing sequence of small cell complexes modeling the dense regions of the data. We test the method on data sets arising in social networks and in image processing. Furthermore, we apply the method to identify new topological structure in a data set of optical flow patches. Joint with Atanas Atanasov and Gunnar Carlsson.
- SIAM Conference on Applied Algebraic Geometry, Oct 2011. [Slides]
- Stanford CompTop Seminar, Jan 2010. [Slides]
- AMS-SIAM Special Session on Applications of Algebraic Geometry, Joint Meetings, Jan 2010.
Topological data analysis: Understanding optical flow. IMA Short Course on Applied Algebraic Topology, June 2009. [Slides]

Software talks and demonstrations

Introduction to Javaplex software for persistent homology. [Abstract]

This exercise session will be a hands-on introduction to persistent homology in topological data analysis. We will use the Javaplex software package, which comes equipped with a tutorial with examples, exercises, and solutions. Two of the real datasets included in the tutorial are the three-circle model for optical image patches, and the conformation space of the cyclo-octane molecule (which is a Klein bottle glued to a sphere along two circles of singularities).
- Applied Algebraic Topology Research Network, Student Online Seminar Series, Oct 2015.
- Young Topologists' Meeting, July 2015.
- AMS Mathematical Research Community on Computational and Applied Topology, June 2011.
Introduction to JPlex software for persistent homology.
- AMS Short Course on Computational Topology, Joint Meetings, Jan 2011. [Slides]
- CSRI Workshop on Combinatorial Algebraic Topology, Aug 2009. [Slides]
- IMA Short Course on Applied Algebraic Topology, June 2009. [Notes]

Departmental talks and Expository talks

Persistent homology, zigzag persistence, and quiver representations, CSU Topology Seminar, Feb 2020. [Abstract]
My talk will be divided into four short vignettes:
1. From a high level I will briefly explain how one can compute persistent homology using Gaussian elimination.
2. We will compute one persistent homology barcode via the more efficient (vectorized) data structure used in software. This improved data structure relies on the fact that boundary matrices are quite structured (and sparse).
3. I will introduce quiver representations and their decompositions.
4. As a specific case of quiver representations, we will learn about zigzag persistent homology and some of its counterintuitive properties.
Neighborly polytopes and the sparsity-promoting L¹ norm, CSU Data Science Seminar, Feb 2020. [Abstract, Notes]

I will survey David Donoho's work connecting the sparsity-promoting L¹ norm to neighborly polytopes. Given an underdetermined system of linear equations y=Ax, it is NP-hard to find a solution x with minimal L⁰ norm, i.e., with as few nonzero entries as possible. By contrast, the convex relaxation of this problem, replacing the L⁰ norm of x with the L¹ norm, can be solved efficiently. Interestingly, for many matrices A, the solution to the L¹ problem and the L⁰ problem coincide if the sparsest solution x is sufficiently sparse. A version of this is true, for example, with random matrices A. I will survey how this "equivalence of L⁰ and L¹ optimization" is related to polytopes, cross-polytopes (L¹ balls), centrally-symmetric polytopes, and neighborly polytopes.
An introduction to Morse Theory. Descriptors of Energy Landscape by Topological Analysis (DELTA), Online Seminar, Nov 2019.
An introduction to applied topology. CSU Data Science Seminar, Oct 2019. [Slides]
An introduction to matroids. CSU Topology Seminar, Sep 2019. [Notes]
Applying to academic jobs. CSU Math Department, Sep 2020.
An introduction to applied topology. Talk for an REU at CU Boulder, June 2019. [Slides]
The waist inequality. CSU Topology Seminar, Mar 2019. [Notes/Slides, Reference]
Lovász' proof of the Kneser conjecture. CSU Topology Seminar, Nov 2018.
Using homotopy colimits to understand infinite simplicial complexes. CSU Topology Seminar, May 2018. [Abstract, Notes]

Last week we gave an introduction to homotopy colimits. Building on this, we will describe how to use homotopy colimits to understand infinite simplicial complexes. The main tools we will describe are the Projection Lemma, the Quasifibration Lemma, and the Cofinality Theorem from Section 3 of Homotopy colimits - Comparison lemmas for combinatorial applications by Welker, Ziegler, and Zivaljevic. The running example we will use is the Vietoris-Rips simplicial complex of the circle.
An introduction to homotopy colimits. CSU Topology Seminar, Apr 2018. [Abstract, Notes]

Though colimits exist in the category of topological spaces, they are often not well-behaved from the perspective of homotopy theory. Homotopy colimits are a tool to fix this. I will give examples of homotopy colimits and describe some of their useful properties. Time permitting, I may describe where homotopy colimits have appeared in my research, or describe their relationship to simplicial sets, or describe their relationship to model categories.
Vietoris-Rips complexes of the circle. CSU Topology Seminar, Oct 2017.
Sperner's lemma and fair division. CSU Math Club, Oct 2017. [Abstract, Notes]

Suppose three roommates need to pay $3,000 dollars of rent per month for their three-bedroom apartment. The three bedrooms are not equivalent: some are bigger, some are smaller, some are closer to the kitchen, etc, and so it's not necessarily fair to have each room cost $1,000. How should the roommates fairly divide the rent between the rooms so that the total rent for the apartment is $3,000, and how should they decide who gets which room? I will describe how Sperner's lemma, a mathematical lemma related to combinatorics and topology, can be used to find a fair division of rent. This topic has appeared in a New York Times article and in a paper by Francis Su.
An introduction to Vietoris-Rips complexes. CSU Topology Seminar, Sep 2017.
Introduction to discrete Morse theory. University of Minnesota Student Topology Seminar, Dec 2013. [Notes]
Applied topology. Stanford University Mathematics Camp Guest Lecture Series, July 2013. [Slides]
Applied topology. Stanford University Mathematics Camp Guest Lecture Series, July 2011.
Coverage problems in sensor networks. Stanford Undergraduate Mathematical Organization Speaker Series, May 2010. [Abstract]

Conferences organized

With Maria D'Orsogna, Rachel Neville, Jose Perea, and Chad Topaz, co-organized an ICERM Topical Workshop on Applied mathematical modeling with topological techniques, Aug 5-9, 2019. [Poster]
With Jeffrey Brock, Melissa McGuirl, Bjorn Sandstede, and Yitzchak Solomon, co-organized an ICERM TRIPODS Summer Bootcamp on Topology and Machine Learning, August 6-10, 2018. Software tutorial available. [Photo]

Mini-symposia organized

With Mikael Vejdemo-Johansson, co-organized an AMS Special Session on Applied Topology, JMM, Denver, CO, Jan 17, 2020.
With Mehmet Aktas, Wenwen Li, Murad Ozaydin, co-organized a minisymposium on Applied and Computational Topology at the SIAM Central States Section Meeting, University of Oklahoma, Oct 5-7, 2018
With Ellen Gasparovic and Katharine Turner, co-organized the 7th Annual Minisymposium on Computational Topology at Computational Geometry Week, Budapest, June 2018.
With Mikael Vejdemo-Johansson, co-organized an AMS special session on Topological Data Analysis at the Joint Mathematics Meetings in San Diego, Jan 2018.
With Florian Frick, co-organized a session on Symmetric Simplicial Complexes and Polytopes at the SIAM Conference on Applied Algebraic Geometry, Georgia Institute of Technology, Jul-Aug 2017.
With Bala Krishnamoorthy, co-organized a session on Recent Advances in Applied Algebraic Topology at the AMS Western Sectional Meeting, Washington State University, Apr 2017.
With Patrick Shipman, co-organized a minisymposium on Applied and Computational Topology at the SIAM Central States Section Meeting, University of Arkansas at Little Rock, Oct 2016.