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Jayadev Athreya's Favorite Theorem

The University of Washington mathematician talks trees, lattices and a plucky constant that seems to show up everywhere

Mathematician Jayadev Athreya is smiling at the camera. He is Indian American, has black hair, and is wearing glasses and a light blue button-front shirt. He is standing in front of a blackboard with math symbols and drawings on it.
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Jayadev Athreya

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American


On today’s episode of My Favorite Theorem, my cohost Kevin Knudson and I were happy to talk to Jayadev Athreya. He’s a math professor at the University of Washington in Seattle. He does research with my spouse, and we are happy to call him and his partner friends. (He mentions an amazing buttermilk pie I made for them one time, probably around 2012. The recipe is here. I skipped the sprinkle of sugar at the end and was not nearly as meticulous as the author when I made the crust.) You can listen to the episode either here or at kpknudson.com, which also has a transcript.

Appropriately for someone living in the Pacific Northwest, Dr. Athreya framed his favorite theorem in the language of trees. If you plant trees in a regular grid pattern and stand at the middle and look out, how many trees can you see? Of course, we need to add a few assumptions. Let’s assume you can see a total distance of R units and that a tree blocks any trees lying directly past it from your point of view, or lying in exactly the same direction from you. The problem can then be restated in less flowery (or leafy) terms as the number of points (m,k) in the plane that are less than R units away from the point (0,0) and where m and k share no common factors. (It's also called the Gauss circle problem.) The answer turns out to be 6/π2, a number that, along with its reciprocal π2/6, shows up many places in math.


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For video introductions to similar tree visibility problems and the number π2/6, check out Numberphile and 3 Blue 1 Brown.

Dr. Athreya explained that this problem, which on its surface seems mostly related to number theory, also connects to geometry and dynamical systems, two of his main areas of research. By accident, this episode ended up being a sort of memorial for three remarkable mathematicians who passed away recently. Learn about Steve Mitchell from his autobiography (pdf) and the blog he wrote starting when he was diagnosed with bladder cancer and continuing until near the time of his death. Learn about William Veech from his Wikipedia page. And learn about Maryam Mirzakhani from the obituary Dr. Athreya wrote for the Indian math journal Bhāvanā, Anton Zorich’s article about her work from 2015 (pdf) after she became the first woman to be awarded the prestigious Fields Medal, or one of the articles collected here by the American Mathematical Society after her death.

In each episode of the podcast, we ask our guest to pair their theorem with food, beverage, art, music, or other delight in life. Dr. Athreya has both a wine and a beer pairing for his favorite theorem. He chose primitivo wine, which as he notes could have been served at the Last Supper, or a spontaneously fermented beer like a lambic. Whether you’re an oenophile or zythologist (which is a real word, red squiggles be darned), he has you covered. You’ll have to listen to the episode to find out why those are perfect accompaniments to lattices, primitive points, and the ubiquitous number 6/π2.

In addition to teaching and research, Dr. Athreya is the director of the Washington Experimental Math Lab, which is one of several geometry labs in universities around the country that do student research and training in addition to math outreach and public engagement. Find out more at their website, WXML, not to be confused with the Christian radio station in Ohio.

You can find more information about the mathematicians and theorems featured in this podcast, along with other delightful mathematical treats, at kpknudson.com and here at Roots of Unity. A transcript is available here. You can subscribe to and review the podcast on iTunes and other podcast delivery systems. We love to hear from our listeners, so please drop us a line at myfavoritetheorem@gmail.com. Kevin Knudson’s handle on Twitter is @niveknosdunk, and mine is @evelynjlamb. The show itself also has a Twitter feed: @myfavethm and a Facebook page. Join us next time to learn another fascinating piece of mathematics.

Previously on My Favorite Theorem:

Episode 0: Your hosts' favorite theorems Episode 1: Amie Wilkinson’s favorite theorem Episode 2: Dave Richeson's favorite theorem Episode 3: Emille Davie Lawrence's favorite theorem Episode 4: Jordan Ellenberg's favorite theorem Episode 5: Dusa McDuff's favorite theorem Episode 6: Eriko Hironaka's favorite theorem Episode 7: Henry Fowler's favorite theorem Episode 8: Justin Curry's favorite theorem Episode 9: Ami Radunskaya's favorite theorem Episode 10: Mohamed Omar's favorite theorem Episode 11: Jeanne Clelland's favorite theorem Episode 12: Candice Price's favorite theorem Episode 13: Patrick Honner's favorite theorem Episode 14: Laura Taalman's favorite theorem Episode 15: Federico Ardila's favorite theorem