Tau Day 2024: Double the DonationHappy Tau Day! 

Many of us are familiar with Pi Day on March 14th, but today we're celebrating Tau Day, June 28th, which playfully celebrates τ = 6.28..., or 2π. 

Because the value of tau is twice the value of pi, supporters of Tau Day call for “twice as much pie." In this spirit, generous SLMath supporters have offered to match your gifts today, 6.28, up to $25,000!

Support SLMath (MSRI) on Tau Day


For Tau Day 2024, we invite you to solve double the puzzles! 
Follow below to play along. 



Puzzle #1: Tau Day 2024 Crossword Puzzle Contest

Puzzle by former SLMath postdoc Melissa Zhang (University of California, Davis)

Thanks to Sejongmall, makers of Hagoromo Chalk, for their generous support.

Preview of crossword puzzle
Puzzle preview

Fill in the puzzle online to win!



To join the Tau Day crossword puzzle contest: 

1) Fill in the puzzle online and use the "Submit Answers" button on the upper left to enter.


2) Print the PDF version at home and snap a photo or scan the completed puzzle. Send your completed puzzle to answers@slmath.org.

Completed entries must be received by 11:59pm Pacific Time on June 28, 2024 to play along!

The first person to submit a correctly solved crossword will receive an official Tau t-shirt (in adult unisex or women's sizes S to 4XL), a box of coveted Hagoromo chalk, and bragging rights as winner of the official SLMath Tau Day medallion. Second place will receive an SLMath tote bag, and third place will receive an SLMath water bottle. (All are welcome to enter the contest, but prize shipments are limited to U.S. addresses only.)
Tau Day 2024 prizes

Thank you to our generous prize sponsors: Sejongmall, Co. Ltd. and Michael Hartl.

Puzzle #2: Celebrate Tau Day with "The Blob"

Created by Peter Winkler (Dartmouth College), author of Mathematical Puzzles: A Connoisseur's CollectionMathematical Mind-Benders​​​, and Mathematical Puzzlesthe Blob is a math puzzle that has yet to be solved.


The Blob puzzle, by Peter WinklerA blob, in the shape of a disk of radius 1, appears on the plane at time 0 and grows in all directions at rate 1 (so that at time 1, if unimpeded, it will be a disk of radius 2). It can be stopped only by a special kind of fence that can be manufactured at rate r (thus, at time t, the total length of all pieces of fence cannot exceed rt.)

What is the least real number b such that if r > b, the blob can eventually be fenced in and the world saved?


Submit your mathematical proof to answers@slmath.org. There is no deadline for this puzzle, but we hope someone might solve it by Tau Day 2025!



Support SLMath on Tau Day


A heartfelt thanks to our generous Tau Day friends. We could not accomplish our goals without their support, involvement, and enthusiasm.

  • ​​​​Michael Hartl
  • David Hoffman & Joan Sarnat
  • Bob Palais, Richard Palais & Chuu-Lian Terng
  • Sejongmall Co., Ltd, makers of Hagoromo Chalk​​​​​

Tau Day is a time to celebrate and rejoice in all things mathematical.
- Michael Hartl

Gifts made on Tau Day - of any amount - allow us to fund scientific programming and workshops, outreach projects, and support for individuals including SLMath Postdoctoral Fellows. The generosity of our donor community allows us to to foster and communicate mathematical research in a broad range of fundamental topics and applications, develop mathematical talent and cultivate a sense of belonging and engagement, and inspire an appreciation of the power, beauty, and joy of mathematics.


Questions? Contact Uta Lorenzen, uta@slmath.org, or use the form below and we will reach out to you directly.