Expository Talks (2011 List)

This is the list of expository talks that I last revised in 2011. While these are less current than the talks on my current expository talks page, I would still be pleased to give most of them at the appropriate venue. I have posted some of the online and printed fliers for previous talks below. The linked "details" page for each talk includes the mathematical level of the talk, the physical requirements of the lecture room, and a list of past venues.

Fortunatus's Purse

Description
Starting with the instructions from a Lewis Carroll novel for making a twisted purse that contains the entire world, this talk takes a scenic tour through the realm of topology. The highlights include non-orientable surfaces, projective geometry, and map-coloring theorems, illustrated with models made of fabric, paper, ceramics, and beads.

The original version of this talk was a 15-minute presentation in the Special Session on Mathematics and Mathematics Education in the Fiber Arts at the Joint Mathematics Meetings in 2005. That lead to my chapter of the same name in Making Mathematics with Needlework: Ten Papers and Ten Projects, and to the expanded talk, which is one of my absolute favorites to give.

Further Details


The Discovery of Non-Euclidean Geometry

Description
This talk evolved from a lecture I gave in graduate school motivated by the pedagogical complaint that we never tell our students the history of the math we teach them. With a mix of slides, posters, and models, this talk meanders through twenty-three centuries of mathematical history. The path is marked by a list of equivalents of Euclid's Parallel Postulate, and the talk concludes with a demonstration that each of these equivalents fails in the non-Euclidean plane.

Further Details

Online Flier for Radical Pi Talk
If you click on “Non-Euclidean Geometry”, you will find Joel Castellanos's package for making compass and straightedge constructions in the Poincare disk. If you click on the blue tiling, you will see an Escher print that I use to illustrate the Poincare disk in this talk.

Printed Flier for Ross Program Talk


The Geometries of Escher

Description
I developed this presentation for the Ohio State University's Undergraduate Recognition Ceremony. The department invited me to give a talk pertaining to the Mathematical Association of America theme for the year, Mathematics and Art. I could not resist returning to my old geometric haunts from “The Discovery of Non-Euclidean Geometry”, and the result is a less comprehensive but more accessible glimpse of the non-Euclidean universe.

After a brief survey of Escher's mathematical and artistic themes, we discuss the polygonal tilings underlying Escher's tesselations, focusing on the triangular reflection tilings. We see that in the Euclidean plane and on the surface of the sphere, there are strong restrictions on the types of triangles that allow reflection tilings, and only finitely many triangular shapes can tile each surface. However, in the non-Euclidean plane, there are infinitely many such tilings. As we go, we see Escher prints and sculptures constructed in all three geometries.

I am indebted to Daniel Shapiro of the Ohio State University for suggesting this focus for the talk.

Further Details


Trisecting the Angle, or Better Math through Cheating

Description
The mathematics behind this talk comes from the discussion of compass and straightedge constructions in What is Mathematics by Courant and Robbins and from the discussion of origami constructions on Thomas Hull's Origami Mathematics Page. After sketching a proof of the impossibility of trisection by compass and straightedge, we perform the Archimedes compass-and-ruler trisection and discuss the mathematics that makes trisection by paper folding possible. At the end of the talk, each member of the audience trisects an angle by origami.

Further Details


Sunflowers and the Least-Rational Number

Description
The central idea of this talk is the mathematical effect of phyllotaxis, the growth process in plants whereby leaves (or seeds or petals) of a plant emerge with a fixed angle between consecutive leaves. We explore why the phyllotaxis angle in sunflower seed heads and in pinecones and in many other plants is the golden secton of the circle and why this angle produces Fibonacci patterns in the spirals of such plants. Continued fractions play a key role in unraveling the mystery.

I also have a more advanced version of this talk for a mathematically savvy audience that I developed as a plenary address at the Young Mathematicians Conference at the Ohio State University, and which ultimately lead to my paper “Dancing Elves and a Flower's View of Euclid's Algorithm.” That talk is available as a streaming video; the PowerPoint slides are improperly synched (but can be advanced manually).

For source materials, see R. Knott's excellent discussion of Fibonacci Numbers and Nature, page 1 and page 2, and the references at the bottom.

Further Details

Printed Flier for Radical Pi Talk

Numbered Pinecones


Hexaflexagons

Description
Hexaflexagons are gadgets invented by the topologist Arthur H. Stone and popularized in Martin Gardner's Scientific American Column. The article appears in his book Mathematical Puzzles and Diversions.

This presentation is more of a workshop than a talk. First, we construct hexaflexagons with three faces and with six faces and learn how to flex them. In playing with the hexahexaflexagon, novices are quickly struck by the difficulty of locating all six faces. We then have a discussion of how to resolve this problem, and if a solution is reached, we discuss how to construct hexaflexagons with different numbers of faces and different internal structures.

Further Details

Online Flier for Radical Pi Talk (with video clip)

Printed Flier for Radical Pi Talk


Crumpling Paper and the Brouwer Fixed Point Theorem

Description
The talk is presented as a wager. I take a sheet of paper, set it on a desktop, mark its position, and have an audience member crumple the sheet and place it within the marks. I then propose the following wager: I will bet any member of the audience that there is a point on the page that lies directly over its pre-crumple position. For the remainder of the talk, I convince the audience that I may collect on this bet.

This talk was inspired by a lecture given by Francis Su (creator of the Math Fun Facts web page) to the Harvard Graduate Student Seminar. His subject was applications of Sperner's Lemma, one of which is a straightforward proof of the Brouwer Fixed Point Theorem.

Further Details

Transparencies for Brown SUMS Talk (HTML)


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