From “God Plays Dice,” an interesting geometry problem
Here's a cute problem (from Robert M. Young, Excursions in Calculus, p. 244): "What is the average straight line distance between two points on a sphere of radius 1?"
(Answer to follow.)
Note the number of comments with different answers.
"Excursions in Calculus," from what I could see at Google Books, looks like it has many interesting problems.
Review: Beautiful Mathematics
Beautiful Mathematics is a collection of interesting mathematical explorations published by the MAA. If you find the following questions (many of which are new to me) intriguing then you'll probably enjoy the book.
- Do you know the dimension of Sirpienski's Triangle?
- Have you ever explored squaring maps?
- What is the Riemann Sphere?
- Can you find a formula that associates Fibonacci numbers and Pi?
- Can a square be inscribed in any triangle?
- What are the first three digits of the millionth Fibonacci number?
- Do you know how to construct a regular heptagon using a straight edge, compass, and angle trisecting device?
- Can you prove Lagrange's Theorem, that every positive integer is the sum of four squares?
- How many triangles are there of perimeter n, where n is an integer, the sides are all integer lengths and the triangles are incongruent?
"Beautiful Mathematics" has nearly 100 challenging investigations, most with elegant solutions presented. Topics include words, images, formulas, theorems, proofs, solutions, and unsolved problems.
The book is available from the publisher in PDF format for half of the price of the hardback book.
Review: Magical Mathematics
Deep Math meets great magic tricks. Magical Mathematics is an absolutely remarkable book. I don't say this lightly. Publishers send me plenty of books to review. Some I like more than others. "Magical Mathematics" is a fantastic book for someone who wants to explore the non-trivial math behind some impressive magic tricks.

While I enjoy purely recreational math puzzles that have no practical application I also love it when I discover challenges that are interesting and relevant. "Magical Mathematics" is chock full of fun (and deep) challenges that students (and adults) can sink their teeth into.
This great review by Sami Assaf at Amazon.com gets to the heart of what makes this book stand apart from many other recreational Math books:
The book is packed with fantastic card tricks that will surely dazzle friends and family (with enough practice), but goes beyond this by explaining the beautiful (often deep) mathematics behind the tricks. The book intersperses magic and mathematics in an engaging way that keeps the reader hooked. The book begins with a simple 4 card trick. Well, simple enough to perform; understanding is a different matter. The authors then explain what mathematical concepts (mostly involving combinations and permutations) are at the heart of the trick, and then generalize the principle involved into a truly impressive, more elaborate card trick. After that, it's back to math to see exactly how and why the magic works. Later chapters follow similar patterns, where the reader is drawn in by a beautiful card trick and the beauty is then heightened with a clear explanation of the underlying mathematics. Along the way, the authors give excellent advice on how often to rehearse the tricks before performing as well as entertaining tips to make for a more engaging performance. The mathematics is presented in digestible bites, with excellent examples and illuminating illustrations.
But be warned: this isn't your simple high school math! Many of these tricks employ sophisticated mathematics using Combinatorics, Group Theory, Graph Theory and more. Fortunately, the authors are adept at explaining these complicated concepts in a clear fashion, but the novice reader may have some trouble following some of the proofs. Hopefully, the reader will be so inspired by the beauty of the subject, that she'll see it as motivation to learn more mathematics! In fact, the authors' unapologetic goal with this text is to corrupt youngsters of all ages into pursuing mathematics in much the same way that the authors themselves were seduced by the subject. Here's hoping they succeed with you as they have with me!
The New York Times recently published a review of "Magical Mathematics." Another review is available at The Math Less Traveled. More information about the book is available at the Princeton University Press website.
Google adds graphing calculator to its search results
Google just announced graphing support to its search engine.

More information at the Google Search Blog.
Hat tip to my brother Abe.
New math book makes its debut on Fibonacci Day
Today is 11/23, which some call Fibonacci Day. I received an email a few days ago from a Mr. Tony Gonzalez who has translated a very popular Japanese math book into English. I did receive a PDF review copy and liked what I saw but will wait to receive a printed copy before reading and reviewing. Here's Tony's email and press release. Tony, I wish you much success.
Hi, Sol.
My name is Tony Gonzalez. I'm a former math teacher (which is how I came to know your blog), but I'm now working mainly as a translator and publisher. I'm writing to let you know about a book that I translated and my company will be publishing next week, "Math Girls". We will be releasing the book on 11/23, "Fibonacci Day", perhaps making it a good topic for a blog post on that day?
I'm taking the liberty of sending you a press release announcing the publication (below). That should give you the rough details, but if you have any questions do please feel free to contact me by email, or you can get more information about the book at our website, bentobooks.com.
Thank you!
―Tony
_______________________________________
Press release
--- For immediate release ---
More fun with the number 11
In honor of 11/22/11 (22 = 11+11), here is a nice complement to my 11/11/11 post.
Happy 11/11/11 Day!
With help from a number of you I produced a screencast and blog article of fun math stuff for 11/11/11.
Read my blog article at the Wolfram Blog.
Another clever exploration by James Tanton
I really enjoy James Tanton's Math explorations because they tend to be easy to describe and rich in exploration value. Here's such an exploration:
The problem statement is very simple. Is there a way in which we can say that there are more triangular numbers than square numbers? If so, how do we compare the sizes of the two sets? Can we compute the ratio of triangular to square numbers where both T(n) and S(n) are less than an arbitrary constant? Can we generalize what we find for other polygonal numbers?
This is a great exploration!
A challenging 11111… puzzle
I received this email from Mr. Brian Silverman who gave me permission to publish it.
I noticed that you were asking for interesting factoids about the number 11.
This isn't quite a factoid about 11 but here's a math puzzle a Russian friend of mine said her son worked out with 7th graders. Seems to me to be beyond most undergrads here. I'm not including the answer, to give you a chance to work on it if you want. but will send it if you ask.
"The question is if it's true that among the numbers consisting of only "1"s (1; 11; 111; 1,111; etc.) there is a number (maybe many) that is divisible by 572,003?
Actually, 572003 is taken arbitrarily. 57 is the number of the school (schools here are mostly numbered instead of having names) and 2003 you possibly know what is recently used for (yes, yes, here in Moscow, too). "
Brian
This puzzle doesn't seem at all obvious to me, especially if one needs to solve it for arbitrary numbers other than 572003. I thought of putting it into my pile of problems to someday solve but then thought it'd be fun to post here.
Can Russian 7th graders solve this? How much help did they get?
Can you solve it?
A great triangle exploration
Mr. Honner has a great exploration at his blog. It starts with a simple question, that has subtlety and depth to it: How do you determine the "equilateralness" of a triangle? Can you compare two triangles and determine which is more equilateral than the other?

The post introducing the investigation is here. I encourage you to do your own exploring before reading the 28 comments which are rich in ideas. Once you've played around with the ideas yourself then take a look at what Mr. Honner came up with in Part II.
I love this kind of exploration for a number of reasons:
- The question is simple to understand.
- Just like in the real world there are multiple approaches.
- It's not clear that there is a right solution but some are better than others.
- Students get to think about properties of triangles in new and different ways.
- Students get to think deeply about the notion of "metric."
- This problem is more interesting than many other geometry problems I've seen.
Nice!