I recently ran across a number of videos I figured would be interesting to regular Grey Matters readers, so I thought I would share them.

We'll start things off with a little math magic, courtesy of Tom London and his appearance on America's Got Talent earlier this week:



Yes, I could explain the method, but I don't want to ruin the fun and the mystery. Just enjoy the magic of the prediction for what it is, since that's how it's meant to be enjoyed.

If you want mathematical explanations, however, I highly recommend checking out PBS Digital Studios' Infinite Series. These are videos on assorted advanced mathematical topics, yet they're taught in a very accessible way. Back in March, I discussed a puzzle which required the understanding of Markov chains to solve. Compare that to their video Can a Chess Piece Explain Markov Chains?, which also happens to employ my favorite chess piece, the knight:



If you enjoy Grey Matters, you may also the work of 4-time USA memory champion Nelson Dellis, who focuses on both mental and physical fitness. He has a series of memory technique videos, as well as interviews with masters of mental skills. Both of these are available on his YouTube channel, as well. As a taste of his skilll, watch his video, Memorizing 28 names in less than 60 seconds!:



Curious how he's able to do that? He explains in the next video in the series, HOW TO // "Memorizing 28 names in less than 60 seconds!".

At this point, I'll wrap things up so you can get started on a potentially mind-expanding journey.

Starting in Delaware, you must tour the 48 contiguous United States, visiting each state exactly once. Where will you finish?

That's a puzzle from Futility Closet. I'll link directly to it later, so as not to get too far ahead. It's really a puzzle within a puzzle, however.

As anyone who has tried the Knight's Tour knows, moving around with the limitation of landing in each space only once can be quite a challenge.

The original Futility Closet puzzle can be solved either by thinking about that puzzle on its own with a little analysis. However, I think it might be a little more fun to try and solve the puzzle by trying various ways to get around the 48 contiguous United States, visiting each state exactly once.

To that end, I've written this as a puzzle you can play below. It's an interactive map, so you can scroll (by clicking and dragging outside of the U.S.) and zoom (using the + and - in the upper left corner of the map) as needed, which helps when you're trying access smaller states.

Sorry about missing a post last week. It turned out to be a busier weekend for me than I originally planned.

I'm back this week, and I've brought plenty of snippets with me to make up for the missing post!

• Back in September, in DataGenetics' Grid Puzzle post, an intriguing puzzle was posed: Imagine there is a grid of squares. You draw a line connecting one intersection to another intersection on the grid. The question is: How many squares does this line cross through?

The link works through the answer, including an interactive widget which may help you work it out. It turns out that the answer involves only simple arithmetic and being able to work out the greatest common divisor (GCD) of 2 numbers.

If you can work out the GCD of 2 numbers in your head, you can turn this into a mental math presentation. How do you go about that? Math-magic.com has the answer, in both webpage form and PDF form! Page 94 of Bryant Heath's Number Sense Tricks also has some great tips on finding the GCD of 2 numbers in your head.

• Also back in September, I wrote about Knight's Tours on a calendar, and included sample calendars for both September and October 2014. I figured it's time for the November 2014 version, included below. This month, 6 is most date-to-move matches I could find:



• OfPad.com seems to have a similar goal to Grey Matters, which is to improve your genius. There's plenty to explore as a result, but I'd like to draw your attention to their Mental Math Tricks posts in particular. There's lots of great tricks, some of which you may not have seen before.

• To wrap up this month's mental math snippets, I'd like to focus on one particular technique for multiplying any two 2-digit numbers together. I've seen this technique before, but the way dominatemath teaches this multiplication technique, it just made so much more sense than the other instructions I've seen for this approach.



That's all for this month's snippets. Have fun exploring them!

As you can probably tell from this recent post and this recent post, I've spent quite a bit of time thinking about the Knight's Tour lately.

These thoughts have reminded of a different type of Knight's Tour puzzle. This unusual variation involves moving the knight around a calendar.

It was 4 years ago, during September or October, that I was looking for blog post inspirations and ran across a thread on the XKCD forums, titled “Knight's Tour revamped”, which suggested playing the Knight's Tour on a calendar.

There was an added challenge, however: With your starting square being considered as move #1, how many dates could you land on that were the same as the move number? For example, if move #1 started on the 1st of the month, both the move number and the date would be 1.

As you can see in the original thread, the original poster used a July 2010 calendar and managed to find a complete Knight's Tour on which the 2nd, 6th, 11th, and 23rd moves landed on the dates of the 2nd, 6th, 11th, and 23rd respectively. Not surprisingly, it was Jaap of Jaap's Puzzle Page who found an 8-match solution.

I filed this in the back of my mind, but never really did anything until I ran across the Solving the Knight’s Tour on and off the Chess Board post which I mentioned last week. I liked the basic idea of being able to input a shape, and have the computer work out the tour, and especially the idea of using it to work out the XKCD forum's calendar challenge.

With a little knowledge of Java and graph theory under my belt, I managed to work out a program to solve it. For my fellow Java programmers, here's the main portion of my program, and here's the KnightsTour class I wrote to support it. Most of the hard work is done by lines 590 to 749. Those and lines 20 to 23 can removed if you're interested more in the general Knight's Tour than the particulars of the calendar challenge.

One of the first things I did, not surprisingly, was to find out how many day-to-move matches I could find in this month's calendar. I also found 8, all of which are highlighted below in red:



Yes, I've gone through every possible calendar, starting on every possible date, and learned quite a few interesting things in the process:

• Due to the fact that the number of days in a week (7) is odd, and the fact that the knight always moves an odd number of spaces (3), this means that a Knight on a calendar will always move from an odd date to an even date, and vice versa (just like what your teacher taught you about adding even and odd numbers). This, in turn, means that it's impossible to get ANY date matches if move #1 begins on an even-numbered date, as all the odd moves will land on even dates, and vice-versa.

• The above fact also means that if you start on an even date in a month with an odd number of days (29 or 31), you won't be able to complete a Knight's Tour.

• Yes, Jaap's 8-match path is the best one possible for July 2010 in particular. It also happens to be the only way to get 8 date-to-move matches in a Knight's Tour of a 31-day month beginning on a Thursday.

• Given any random month and year, you can always find a complete Knight's Tour and at least 6 date-to-move matches. Surprisingly, these minimum matches aren't found in the shortest months, as you may expect. With 30- and 31-day months starting on a Saturday, as well as 31-day months beginning on a Friday, 6 is the highest number of date-to-move matches you'll be able to find.

• There are months with 9 date-to-move matches, but none with more than that. 9 date-to-move matches can be found in a 29-, 30-, or 31-day month starting on a Tuesday or a Wednesday. In a 29-day month starting on a Thursday, or a 31-day month starting on a Monday, you can also find 9 date-to-move matches. You can often find more than 1 way to get to these matches, as well.

As it happens, next month (October 2014) is a 31-day month starting on a Wednesday, and here's one of the 3 possible ways to get 9 date-to-move matches:



I chose this one simply for the elegance of the column containing 16-23-30 and the diagonal containing 12-20-28. I also find it interesting that so many powers of 2 have date-to-move matches (2-4-8-16).

For the more math-inclined geeks, I'll wind this post up with all the maximum number of matches I've found, including the dates on which they start:

28-day months, starting on:
Sunday:    7 matches, beginning from the 1st or 23rd
Monday:    7 matches, beginning from the 1st or 21st
Tuesday:   8 matches, beginning from the 25th
Wedneday:  8 matches, beginning from the 1st
Thursday:  8 matches, beginning from the 1st or 5th
Friday:    8 matches, beginning from the 1st or 15th
Saturday:  7 matches, beginning from the 23rd or 25th

29-day months, starting on:
Sunday:    7 matches, beginning from the 1st
Monday:    7 matches, beginning from the 1st or 27th
Tuesday:   9 matches, beginning from the 25th
Wedneday:  9 matches, beginning from the 1st or 11th
Thursday:  9 matches, beginning from the 1st
Friday:    7 matches, beginning from the 1st, 5th, 27th, or 29th
Saturday:  7 matches, beginning from the 1st

30-day months, starting on:
Sunday:    7 matches, beginning from the 7th or 23rd
Monday:    8 matches, beginning from the 27th
Tuesday:   9 matches, beginning from the 25th
Wedneday:  9 matches, beginning from the 11th
Thursday:  8 matches, beginning from the 1st
Friday:    7 matches, beginning from the 1st or 7th
Saturday:  6 matches, beginning from the 1st or 25th

31-day months, starting on:
Sunday:    8 matches, beginning from the 23rd
Monday:    9 matches, beginning from the 7th or 31st
Tuesday:   9 matches, beginning from the 1st, 23rd, or 25th
Wedneday:  9 matches, beginning from the 7th
Thursday:  8 matches, beginning from the 5th
Friday:    6 matches, beginning from the 1st, 5th, 7th, or 31st
Saturday:  6 matches, beginning from the 1st, 23rd, 29th, or 31st

Since I've changed my posting schedule, I seem to have neglected my monthly snippet posts!

Not to worry, however, as we're kicking off September with a good round-up of different takes on some of my favorite mental feats.

• One of the longest-standing tutorials on Grey Matters is the classic Knight's Tour. The traditional version usually happens on an 8 by 8 chessboard. What about other irregular, non-rectangular shapes?

Over at the Wolfram Blog, Jon McLoone explored that question using Mathematica in his post Solving the Knight’s Tour on and off the Chess Board. If you're interested in the programming and the math, there's plenty in this article. Even if you don't care for all the math and programming, the variety of boards with successful Knight's Tours is amazing and amusing. Who knew Pac-Man could play the Knight's Tour so well?

• Over in the Mental Gym, I have a full tutorial on squaring 2-digit numbers in your head. I've often wanted to move on to squaring 3-digit numbers, but never really found a method that suited me. However, I recently ran across a video tutorial from Mind Math called Mental Math Trick to Square 3-digit Numbers for Faster Calculation. It breaks the problem up into 2 steps, working with the hundreds digit followed by the remaining 2 digits as a group. If you're used to squaring 2-digit numbers, this method isn't difficult to learn and adapt:



• Back in March, I wrote a post about calculating powers of e in your head. At the time, I was unaware of Colin Beveridge's post, Secrets of the Mathematical Ninja: Estimating Powers of e, which featured a quicker, yet less accurate estimate.

After seeing my post, Colin took it upon himself to develop an improved method, which he posted as Powers of e Revisited: Secrets of the Mathematical Ninja. When you're done exploring those posts, check out the rest of Colin's Blog!

• Another favorite blog topic of mine is calendars. Beyond the standard day of the week for any date feat, there's plenty of interesting mathematical patterns and shortcuts waiting to be discovered in the calendar. One of the best round-ups I've found on the internet is P.K. Srinivasan's Number Fun with A Calendar (PDF version). Besides the PDF version, there's a zipped .DOC version and even a video demonstration of some of the topics from the book:



That's all for this month. I hope you found these enjoyable and useful!

One of my favorite mental challenges, as many regular Grey Matters readers know, is the Knight's Tour. The challenge is, using only the chess knight's L-shaped move, to land on each of the 64 squares once.

Mentalist Richard Paddon recently released a download resource titled The Knight's Tour: A Scenic Journey. In this post, I'll take a close look at this new take on a classic feat.

We'll start with a quick peek at Richard Paddon himself performing the Knight's Tour, via the teaser ad:



The Knight's Tour: A Scenic Journey comes as a set of 3 files: A 45-page PDF of the same title, a 16-minute MPEG file of Paddon's performance, and the Knight's Tour Windows application used by Paddon, and programmed by Dave Everett.

In the PDF, right away, the author emphasizes the importance of developing drama in the Knight's Tour presentation. The first parts of the actual instruction, however, focus on developing the path through the board. Much of this part of the book may be familiar to readers of the “Knight's Tour” section of Paul Brook's Chrysalis Of A Polymath. However, Richard Paddon does add some new and helpful notes, such as the section on what he has dubbed “delta values”, which are familiar to those who have programmed a Knight's Tour, but little discussed in the use of performances.

In the next half of the book, Paddon discusses the presentational details. He starts with the benefits of the Knight's Tour, including its uncommon nature, and its huge potential on an emotional and theatrical scale. The thoughts behind the presentations are well laid out. Even if you disagree with any aspect of the presentation as written, you at least have a good starting point of why particular choices were made.

One of the more interesting choices is ending on a selected square, as seen in the above video. As the board empties, the chosen square becomes a more and more important focus, and becomes a natural point of building tension. The PDF winds up with a detailed description of how to use the program.

There are very few weaknesses in this product overall. One of the one that stands out to me as both a programmer and a blogger of mental feats was the choice of the Comic Sans font for the numbering of the board. If you're taking as much care as this author does to make an impact on the audience, there's probably better ways to label your board than a font designed specifically to have a comic-book appearance. On an equally minor note, the lightning in many of the shots of the performance video could be better. The importance of the video is for a more complete understanding of the presentation, so this isn't a huge drawback.

Overall, this is an excellent value for anyone seriously interested in performing the Knight's Tour. The basics of working through the path may be easily accessed in multiple sources, but the depth of knowledge that is presented, as well as the use of multiple media to demonstrate this make this the most complete lessons about all aspects of the Knight's Tour and its proper performance.

It's available for only $9.95 over at Lybrary.com and is a remarkably great value for that money. If the Knight's Tour interests you, Richard Paddon's The Knight's Tour: A Scenic Journey is a must-read.

I love it when two old friends visit and get along!

Numberphile recently took a look at one of my favorite challenges: The Knight's Tour! This is a chess-based puzzle that challenged some of the greatest minds in mathematics.

In a rare video appearance, Brady himself describes some of the fascinating aspects of the Knight's Tour in the following video:



If you'd like to try the Knight's Tour out for yourself online for free, I've created 2 versions you can play. This version is in the Mental Gym (select level 1 for the classic challenge), and here's a more modern version hosted on Dropbox (select “New Game” > “All 64 Squares” for the classic challenge).

When it's a new challenge, it can seem quite difficult. Often, you get past about 50 squares, and then start having difficulty. If you want to be able to tackle this challenge, I have provided a complete Knight's Tour tutorial over in the Mental Gym. If you can understand and remember a few simple patterns, you can not only solve the Knight's Tour starting from anywhere, you can even have someone select a starting AND ending position, and still be able to solve it!

My dropbox version of the Knight's Tour offers various settings, including the ability to show a numbered path, as in the video. This version also auto-detects whether the numbered path is a semi-magic square, as discussed in the video, starting at about the 2:23 mark.

If you can learn to solve it, as in the Mental Gym tutorial, is it possible to learn to start anywhere and create a semi-magic knight's tour square? The answer is almost. Magician Harold Cataquet has done some incredible work on working out just how to do this, and it's written up in the ebook Mind Blasters, by Peter Duffie. If you're really interested in being able to the Knight's Tour AND finishing with a semi-magic square, the article is worth the price of this one book alone.

As Brady mentions in the video, there's an amazing amount of mathematical research done on the Knight's Tour. You can see many of the directions in which this challenge was taken over at Knight's Tour Notes, for a start.

Play around and enjoy the Knight's Tour. If you have any interesting discoveries you'd like to share, I'd love to hear about them in the comments!