Volume 8, Number 4 of the Bulletin kicks off with the concluding half of our 2-part interview with Hamilton College Assistant Professor of Mathematics Courtney Gibbons. In this second half, one of the objects she describes are the Cayley graphs of groups. This inspired the creation of several Cayley graphs by members of Girls’ Angle, which are featured in this issue’s Math Buffet.

The cover itself also shows a Cayley graph of .

Next, Emily and Jasmine continue their exploration of *n*-pointed stars. This time, they follow-up on an observation that Emily made about the (17, 7)-star on the cover of Volume 8, Number 3. She noticed that the (17, 7)-star contains (17, *k*)-stars for *k* = 1, 2, 3, 4, 5, and 6. Together, they not only show that this is an instance of a general phenomenon, but they also find the relationship between the sizes of such embedded stars within stars.

Anna wraps up her study of cross sections of the surface *z* = *xy*.

In Part 3 of The Derivative, we explain both the Chain Rule and the Quotient Rule along the line emphasized in this series: local linearity. We hope that the Chain Rule, especially, appears “obvious” from this point-of-view. For another take on the Chain Rule, here’s an earlier post that uses movies to explain it.

Next, Stuart Sidney, Emeritus Professor at the University of Connecticut, entertains us with curious facts about palindromic numbers.

We introduce a mathematical variant of the classic Hot Potato game that can be used as a vehicle to explore quite a few math concepts and in quite a bit of depth.

Finally, in Notes From the Club, we give a brief account of Bathsheba Grossman’s recent Support Network visit to Girls’ Angle. If you haven’t seen her mathematically inspired sculptures, you’re in for a treat. She mesmerized us for over an hour with her inspiring creations.

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!

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We claim that every point in space on the side of the plane with the rolling snowball and within a snowball diameter of the plane can be visited by the marked point.

Let *C* be the circumference of the snowball. Any positive distance less than 2*C* can serve as the base of an isosceles triangle with equal sides of length *C*. Draw this triangle in the plane and put the snowball at one endpoint of the base with its marked point touching the plane. Roll the snowball along the two sides of length *C*. It ends up at the other endpoint of the base with its marked point touching the plane. This shows that the marked point can visit any point within a circle of radius 2*C* from its starting point. By taking several such journeys, we see that every point in the plane can be visited. For any point *P* on the same side of the plane as the snowball and within a snowball’s diameter of the plane, position the snowball so that its marked point is at *P*. We can roll this snowball until the marked point is touching the plane and since all points in the plane are visitable, we conclude that *P* can also be visited.

We claim that the locus of visitable points is a cardioid of revolution.

Let’s mark the point on the sphere where the marked point on the snowball touches it at the start with a golden dot. Look at the situation from the point-of-view of the common tangent plane. The two balls roll as mirror images of each other. The marked points are reflections of each other in the common tangent plane.

In other words, no matter how the snowball rolls about the sphere, the marked point will be located at the reflection of the golden dot in the common tangent.

We conclude that the locus of visitable points is a surface with rotational symmetry (about the axis one of whose poles is the golden dot). A vertical cross section by a plane containing the pole of the stationary sphere is the curve traced out by a dot on a circle as it rolls about a circle of the same diameter, and this is known to be a cardioid.

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Last Sunday, Girls’ Angle hosted a booth at the Girl Scouts STEM Expo in Framingham, Massachusetts. Visitors were challenged to accomplish 3 tasks, one of which was solving this crossword puzzle. They succeeded, so now we’re opening this puzzle to the general public. Solve this crossword alone or with friends and relatives. Scan in or take a digital photo of your completed grid and send it to girlsanglepuzzler “at” gmail.com. We will randomly draw from the correct answers received by midnight on **April 4, 2015** to select a “winner” and send the winner a small prize. Girls’ Angle members will be put into a separate pool for a different prize. (If you worked on this puzzle at the Girl Scouts STEM Expo, you can’t enter this raffle. Sorry!)

Across

1. Prefix meaning

5. Soften with warmth

9. Laboring train sound

13. Tip

14. Something to follow

15. Pool division

16. Even

19. Temporary cut protector

20. Like mathematical concepts

21. Dr. Seuss’s ___-I-Am

24. Teach

27. Steal

29. Make collinear

31. What a ghost might say

32. 2012 Ben Affleck film

33. A fact only you know

35. Opposite of SSW

37. They used shells, dots, and bars for numbers

38. Slope

41. A failure

43. Something you hack with

44. Like a proper subset of a set

47. Darkens

48. Say “pi is equal to 3.14.”

50. Turns

51. Mimic

52. An example of something by Bach

55. Erase (abbr.)

56. Jack climbed one

58. Baby’s first utterance?

60. Line

66. Circle, square, or triangle

67. Fencing tool

68. Ready

69. Rugged cliff

70. Men with children

71. Major tennis tournament

Down

1. A little bit

2. Precedes graph, cycle, or gram

3. Depress the accelerator

4. It’s often labeled with *x*, *y*, or *z*

5. A group of three

6. Noisy mob

7. Can be found at the end of some scores

8. Tiny

9. Bonnie’s partner

10. Dislike intensely

11. Not justified

12. Earth science

17. Parts of an act

18. Smallest amount of computer memory

21. Name for a congruence theorem

22. Pub drink

23. Very small alien rock

25. Change a jpeg to a gif

26. Top quality

28. Big strangler

30. Helps you hold on to things

32. Entertain

34. It can be green or black

36. Kind of eagle

39. They’re often yoked

40. Make new and improve

41. They recommend how much to eat

42. Mouth along to music

45. Poetic before

46. Broadband choice

49. Smashed into

52. Held on tightly

53. Abbreviation often found on police rap sheets

54. Drains

57. Blue green

59. The Matterhorn is one of them

61. Type of light (abbr.)

62. Place to unwind

63. Address on the net

64. Precedes horse, cucumber, and lion

65. Unit for pulse

Good luck!

(We will not use your contact information for any purpose other than to deliver your prize, should you win. After the winner has been selected, all emails received will be promptly deleted. At the winner’s discretion, we will let you know who won. Anyone who makes more than one submission will be disqualified! Sorry! Also, this offer is only valid in those states in the United States where such things are legal. There is no fee to enter this puzzle contest.)

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Volume 8, Number 3 of the Bulletin kicks off with the first part of a 2-part interview with Hamilton College Assistant Professor of Mathematics Courtney Gibbons. In this first half, she discusses how she got interested in math, tells us about how she learns, studies, and creates math, and explains the notions of “fixing set” and “fixing number.” She also tells us about some of her goals as a mathematician.

The best way to learn math is to do lots of problems: after all, that’s what math is!

– Courtney Gibbons

Next, Emily and Jasmine continue their exploration of *n*-pointed stars, or, more accurately, convex (*n*, *k*)-stars. In this installment, they succeed in finding and proving a formula for the sum of the tip angles of such stars. The cover of this issue shows examples of (17, *k*)-stars. If you can find a good friend to explore mathematics together with, it can be a lot of fun and quite rewarding. What do you notice about the (11, 5)-star at right?

Next up, our second installment of Meditate to the Math which features math related to cutting a chocolate bar in half along one of its diagonals.

Anna continues her investigation of cross sections of the graph of *z* = *xy* and finds a cool family of hyperbolas.

Konstanze Rietsch of King’s College London contributes a curious number puzzle borrowing characters from Lewis Carroll to state it.

In our second installment of our mini-series on the derivative, we explore basic properties of the derivative and then apply them to deduce the derivatives of any polynomial.

This issue’s Learn by Doing explains two different ways to find all primitive Pythagorean triples and then relates the two methods.

What we hope is that the Bulletin induces you to *do math*. That’s why this issue is filled with math questions and content that shows how others do math.

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!

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Both problems involve a snowball in the shape of a perfect sphere. Mark a point on the surface of the snowball. The snowball rolls without slipping. Also, there is no unnecessary rotation. (In other words, the snowball rotates only around an axis perpendicular to the plane of the great circle that is tangent to the path that the snowball travels.) In each problem, the snowball starts with the marked point touching the surface it is rolling on.

**Snowball Problem 1**. If the snowball rolls about on a plane, what is the locus of all the points in space that the marked point can visit?

**Snowball Problem 2**. If the snowball rolls about on another sphere of the same radius, what is the locus of all the points in space that the marked point can visit?

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After reading *Stained Glass Angles* in Volume 7, Number 4, Marion Walter happened upon an amazing stained glass window by John Rose at the Eugene Public Library in Oregon. That inspired her to suggest a *Math Buffet* column featuring mathematically inspired stained glass windows. One thing led to another, and in this issue, we feature stained glass windows from 7 designers. The cover shows a detail of a stained glass window designed by Millie Wert, a graduate of Harpeth Hall. (Added January 27, 2015: For more math inspired stained-glass windows by students of Thaddeus Wert at Harpet Hall, please visit his blog.)

Next, comes the second half of Margo Dawes’ interview of Cathleen Morawetz, Professor Emerita at the Courant Institute of Mathematical Science at New York University.

Emily and Jasmine continue discussing *n*-pointed stars. Last issue, they showed that the sum of the tip angles of any 5-pointed star is 180 degrees. This time, they try to generalize to *n*-pointed stars, like the 7-pointed star at right. They end up in a discussion about definitions: just what is an *n*-pointed star anyway?

Pull out your compass and straightedge for this issue’s *Learn by Doing*. All of our *Learn by Doing* installments offer the opportunity to explore math through problem solving. The problems start out with the aim of gaining familiarity with the topic at hand and gradually probe deeper and become more challenging. Can you make it all the way through? If not and you’re a subscriber, don’t hesitate to email us for help.

Anna had so much fun exploring cross sections of a paraboloid of revolution, she decided to investigate cross sections of the graph of *z* = *xy*.

If you want to tell us anything about your mathematical explorations, we encourage you to email us about it.

In this issue, we’re beginning a mini-series on the derivative. There are many fine books and textbooks devoted to the subject of calculus. For example, there’s Courant and John’s *Introduction to Calculus and Analysis, Volume 1,* Rudin’s book *Principles of Mathematical Analysis*, and many, many more. Instead of attempting to produce our own textbook, we’re going to give a non-rigorous, but hopefully illuminating explanation. If one grasps local linearity, many results in calculus can be intuited and we hope to show how.

There’s more, including: a card game we call “Full Deck” that addresses the critical technical skill needed to learn how to multiply and divide with decimal numbers, an introduction to the choose function notation, and Notes from the Club where you can read a summary of Prof. Cornelia Van Cott’s recent visit to the club.

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!

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At Girls’ Angle, we do love digits!

Solve this crossword alone or with friends and relatives. Scan in or take a digital photo of your completed grid and send it to girlsanglepuzzler “at” gmail.com. We will randomly draw from the correct answers received by midnight on **January 4, 2015** to select a “winner” and send the winner a small prize. Girls’ Angle members will be put into a separate pool for a different prize.

*This crossword puzzle is dedicated to Connie Chow.*

Across

1. Rest

6. Fall mo.

9. Sub

14. Breathing

15. Greek letter

16. Loaded with marinara

17. Hushed

18. Goal

19. Vermeer product

20. Sunrises and hatchings

23. Soft cheese

24. It often gets beat

25. Rights wrongs

27. City in 54D

31. Donor

33. Unlike Smith

34. Acts like a ghost

36. Where cultures are made

39. Für Elise form

41. Type of health plan: Abbr.

42. Rise over run

44. Increases

45. Forest and jungle

48. Divisible by 2

49. Catches up to

50. Most frigid

52. Triangular sail, or, a 15-year-old who lives near the *Endeavour*?

55. Hamburg’s commission

56. Drive the getaway car

57. Equal areas in equal times

64. Burdens

66. Overly

67. Baking bean

68. Bad color combination

69. Trig function

70. Seaweeds

71. Veracity

72. Time unit’s plural form: Abbr.

73. Name with a double consonant

Down

1. Docile

2. Graduate

3. Flying diamond

4. Follows for or what

5. Pentagon property

6. Type of ape

7. Elegant

8. *The Princeton Companion to Mathematics*, for example

9. Hawaii is the last of these: Abbr.

10. Samson’s bane?

11. No longer useful

12. Corrosive

13. Kids

21. They can be bruised

22. Oft used file command

26. Place to eat

27. Brown

28. Continuous image of the circle

29. Writes

30. Total

31. Most exciting match

32. Fe

35. Soften in the mouth

36. Zero to Federer

37. What cosine does to sine

38. Not straight

40. Shrek

43. Hawaiian graduation gift

46. File’s partner

47. Tails

49. Loathe

51. Someone you only drive to work with?

52. Fastener

53. Haliotis

54. State of 27A

55. Clean with string

58. Draw

59. Low quality

60. Shelf with a view

61. Plastic brick

62. Not home

63. Gave more importance

65. Short

Good luck!

(We will not use your contact information for any purpose other than to deliver your prize, should you win. After the winner has been selected, all emails received will be promptly deleted. At the winner’s discretion, we will let you know who won. Anyone who makes more than one submission will be disqualified! Sorry! Also, this offer is only valid in those states in the United States where such things are legal. There is no fee to enter this puzzle contest.)

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This time, we’re leaving it up to readers to guess what our latest cover represents.

This past summer, Girls’ Angle program assistant Margo Dawes traveled to New York City to interview Cathleen Morawetz, Professor Emerita at the Courant Institute of Mathematical Science at New York University.

Last year, Professor Jennifer Roberts, the Elizabeth Cary Agassiz Professor of the Humanities at Harvard University, wrote an essay for Harvard Magazine about “creating opportunities for students to engage in deceleration, patience, and immersive attention.” Inspired by that essay, and recognizing that this could be a valuable exercise in mathematics as well, we attempt to give you such opportunities in mathematics with our new column, *Meditate to the Math*. Our first installment features the 9-point circle. Instead of reading about the 9-point circle, we encourage readers to find a comfortable, quiet place, and contemplate a geometric figure. We hope this will be a way to take part in the process of mathematical discovery.

Next, follow Emily and Jasmine as they contemplate 5-pointed stars. If any of our members or subscribers have an exciting story of mathematical discovery of their own, we welcome you to tell us about it!

This issue’s *Learn by Doing* addresses quadratic residues. Last Summer’s batch of Summer Fun Problem sets included one on quadratic reciprocity by Cailan Li. But before quadratic reciprocity, there are lots of things to say about quadratic residues. We explore some of those neat properties here.

Last issue, Anna made a neat discovery about stereographic projection and paraboloids of revolution. As often happens with mathematical theorems, the first proof is messy and then spiffier proofs are found later. In this issue’s *Anna’s Math Journal*, Anna finds a much nicer proof and then applies the result to describe a few more observations about paraboloids of revolution.

While contemplating paraboloids of revolution, Anna also came upon a way to understand the radical axis of two circles. This observation seemed more convenient to write an article on because she came to this understanding without writing anything down. She explains in *Seeing the Radical Axis*. Lightning Factorial supplements her article by briefly defining the radical axis for readers not yet familiar with the concept.

A few weeks ago at the Girls’ Angle club, some members helped to simplify Lunga Lee’s excessively long descriptions of various functions. You can try your hand at this in *Function Madness*.

Also inside are another installment of *Math In Your World*, some exercises about real algebraic varieties (to follow-up on Dr. Zamaere’s introduction of them in her interview in the previous issue), and some notes from the club, which include a summary of Emily Pittore’s recent visit. Emily is a robotic vision engineer from iRobot, the maker of the Roomba vacuum cleaner.

We hope you enjoy it!

*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!

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Take a multiplication table and build a column of cubes over each entry. For each column, use as many cubes as the product it sits upon. The result is a “multiplication sculpture” or “multiplication tower.” The picture shows a 15 by 15 multiplication sculpture built by Jane Kostick in 2008. For more examples, check out Maria Droujkova’s photo collection at moebius noodles.

Last spring, club members at Girls’ Angle built a 4 by 4 version out of cubes that were 3 inches on a side. They thought about its properties, such as how many cubes make up an *N* by *N* multiplication sculpture.

We’re going to address this last question and end up with a nice, concrete proof of a famous algebraic identity.

Over the *x*, *y* entry, there are *xy* cubes, so we have to add up all products *xy* where *x* and *y* range over the values from 1 to *N*, and this totals

.

Wait! Where else does that expression occur?

It is also the sum of the first *N* (positive) perfect cubes! In math notation,

.

The implication is that the number of cubes sitting over the last row and column of the multiplication table must be . If we can show this directly, we’d have a nice, concrete proof of the sum of cubes formula.

The columns of cubes over the last row of the multiplication table form a staircase with steps of height *N*. Saw these *N* columns off and get a flat, staircase-shaped plank, *N* cubes long and cubes high. Next, saw off the columns of cubes over the last column (of the multiplication table). You’ll get an almost identical staircase-shaped plank. The only difference is that the last step of total height is gone because it was removed when the columns over the last row (of the multiplication table) were sawed off.

Turn one of these planks over, and the two planks will fit together perfectly to form an by rectangle, and an by rectangle has cubes in it!

Thus, a secret key to the identity

is hidden in plain sight in the multiplication table that many of us learned in elementary school!

To read about how Jane made this 15 by 15 multiplication sculpture, and see hints about more of its properties, check out the December, 2008 issue of the Girls’ Angle Bulletin, pages 12-14 and 25-28.

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