On the television show *The Big Bang Theory, *physicist Sheldon Cooper claimed that 73 is the best number. In the November issue of *Math Horizons* Jessie Byrnes, Chris Spicer, and Alyssa Turnquist investigate Sheldon's claim and leave the mathematical community with a new conjecture about prime numbers. Brian Lins explains how he used mathematics to make his car pool both easy to manage and fair to all drivers. Students: Are you interested in doing mathematics this summer? We've got some ideas for you: five students write about their summer mathematical adventures. Our Do the Math! series continues with Laszlo Bardos presenting several ways to make a torus out of paper. Enjoy these articles and more in this issue of *Math Horizons.* —Dave Richeson

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## Articles

### A Toast! To Type 15!

Katharine Merow

Three researchers discover a new pentagonal tiling of the plane.

### Making a Hash of Things

Adam A. Smith and Ursula Whitcher

Use a hash function to protect stored passwords.

### The Sheldon Conjecture

Jessie Byrnes, Chris Spicer, and Alyssa Turnquist

In The Big Bang Theory, Sheldon asserts that 73 is the best number.

### Whose Turn Is It to Drive Today?

Brian Lins describes a fair and convenient method of managing a car pool.

### Make Math Your Summer Fling

Alissa S. Crans and Beck Weinhold

Five students describe their mathematical summers.

## Do The Math!

### How to Make a Torus

Laszlo C. Bardos shows various ways of making a torus out of paper. (pdf)

Constructing the Miyaoto-Moneroa-Monterde model of a torus: printable template and video instructions.

Constructing a paper torus with seams along Villarceau circles: printable template and video instructions.

## The Bookshelf

Theron J. Hitchman reviews *Things to Make and Do in the Fourth Dimension*, by Matt Parker;

Heather Moon reviews *Single Digits*, by Marc Chamberland.

### 10 Life Lessons from Differential Equations

John D. Cook

What can we learn about life from the study of differential equations?

### The Power of Two . . . in Poetry?

Amy Shell-Gellasch and J. B. Thoo describe the mathematics in Indian poetry.

## The Playground

The *Math Horizons* problem section, edited by Gary Gordon

### Aftermath: The Common Core for Mathematics in a Nutshell

Eve Torrence argues that we can get students hooked on mathematics using art.