## Girls’ Angle Bulletin, Volume 11, Number 5

This issues’ interview is with Duke University Assistant Research Professor Heekyoung Hahn. Prof. Hahn had a remarkable journey from a rural farm in South Korea to mathematician. For most of her student years she would walk an hour each way to school. She had no toys. Instead, she would spend hours playing with math. Today, not only does Prof. Hahn play with math as a professional researcher, she also created a summer program for high school girls called SWiM.

Deanna Needell’s third installment to her column The Needell in the Haystack is about classification algorithms. How does an ATM machine read the checks you deposit into it to determine the correct amount of the deposit? It must look at the often handwritten digits and decide which digit each is. That is an example of a classification algorithm. In this case, written symbols are classified as various numerical digits. She concludes by describing some of the latest work she’s done with her colleagues on the topic. These days, the importance of such algorithms cannot be overstated.

Anna solves the math problem she began investigating two issues ago: Reduce Pascal’s triangle modulo 3 and interpret each row as a ternary number. In this way, one obtains a sequence that begins 1, 4, 16, 28, 112, 448, etc. How many of the first 2018 of these numbers are odd? This was one of the problems at this year’s SUMIT 2018.

Emily and Jasmine figure out a nice shape into which circles with radii in arithmetic progression can neatly be stacked. For a hint as to what this shape is, take a look at this issue’s cover.

Our June/July issue also contains this year’s batch of Summer Fun problem sets. As Prof. Hahn said in her interview, “Math is not something to be afraid of. Rather, it is something that you should enjoy playing with.” The Summer Fun problem sets provide an opportunity for you to play with math. This year, we have contributions from Texas A&M undergraduate Whitney Souery who is currently enrolled in Harvard’s Neonatology Summer Student Research Program at Boston Children’s Hospital and Beth Israel Deaconess Medical Center, two Harvard undergraduates, Laura Pierson and Vicky Xu, and one from newly minted Princeton PhD Matthew de Courcy-Ireland. They provide a wealth of problems to start playing with math on topics ranging from generating functions, step functions, probability, and Markoff triples. As with all Summer Fun problem sets, all members and subscribers to the Bulletin are welcome to send in their solutions and comments!

Also in this issue, a Meditate to the Math on Napoleon’s theorem and some Notes from the Club featuring the second end-of-session Math Collaboration created by mentors at Girls’ Angle, this one made by Northeastern graduate student Jacqueline Garrahan and MIT undergraduate Elise McCormack.

We hope you enjoy it!

Finally, a reminder: when you subscribe to the Girls’ Angle Bulletin, you’re not just getting a subscription to a magazine. You are also gaining access to the Girls’ Angle mentors.  We urge all subscribers and members to write us with your math questions or anything else in the Bulletin or having to do with mathematics in general. We will respond. We want you to get active and do mathematics. Parts of the Bulletin are written to induce you to wonder and respond with more questions. Don’t let those questions fade away and become forgotten. Send them to us!

We continue to encourage people to subscribe to our print version, so we have removed some content from the electronic version.  Subscriptions are a great way to support Girls’ Angle while getting something concrete back in return.  We hope you subscribe!