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kinights tour 3

The Knight’s Tour is a mathematical puzzle that has endured over 1000 years.  The question is simple enough – a knight (which can move as illustrated above) wants to visit all the squares on a chess board only once.  What paths can it take?  You can vary the problem by requiring that the knight starts and finishes on the same square (a closed tour) and change the dimensions of the board.

The first recorded solution (as explained in this excellent pdf exploration of the Knight’s Tour by Ben Hill and Kevin Tostado) is shown below:

knights tour 4

The numbers refer to the sequence of moves that the knight takes.  So, in this case the knight will start in the top right hand corner (01), before hopping to number 02.  Following the numbers around produces the pattern on the right.  This particular knight’s tour is closed as it starts and finishes at the same square and incredibly can be dated back to the chess enthusiast al-Adli ar-Rumi circa 840 AD.

Despite this puzzle being well over 1000 years old, and despite modern computational power it is still unknown as to how many distinct knight’s tours there are for an 8×8 chess board.  The number of distinct paths are as follows:

1×1 grid: 1,
2×2 grid: 0,
3×3 grid: 0,
4×4 grid: 0,
5×5 grid: 1728,
6×6 grid: 6,637,920,
7×7 grid: 165,575,218,320
8×8 grid: unknown

We can see just how rapidly this sequence grows by going from 6×6 to 7×7 – so the answer for the 8×8 grid must be huge.  Below is one of the 1728 solutions to the 5×5 knight’s tour:

knights tour 5x5

You might be wondering if this has any applications beyond being a diverting puzzle, well Euler – one of the world’s true great mathematicians – used knight’s tours in his study of graph theory.  Graph theory is an entire branch of mathematics which models connections between objects.

Knight’s tours have also been used for cryptography:

kinights tour 2

This code is from the 1870s and exploits the huge number of possible knight’s tours for an 8×8 chess board.  You would require that the recipient of your code knew the tour solution (bottom left) in advance.  With this solution key you can read the words in order – first by finding where 1 is in the puzzle (row 6 column 3) – and seeing that this equates to the word “the”.  Next we see that 2 equates to “man” and so on.  Without the solution key you would be faced with an unimaginably large number of possible combinations – making cracking the code virtually impossible.

If you are interested in looking at some more of the maths behind the knight’s tour problem then the paper by Ben Hill and Kevin Tostado referenced above go into some more details.  In particular we have the following rules:

An m x n chessboard with m less than or equal to n has a knight’s tour unless one or more of these three conditions hold:

1) m and n are both odd
2) m = 1, 2 or 4
3) m = 3 and n = 4,6,8

Investigate why this is!

If you enjoyed this post you might also like:

e’s are good – He’s Leonard Euler. A discussion about the amazing number e and Euler’s use of graph theory.

Sierpinski Triangles and Spirolateral Investigation Lesson Plan. A lesson to introduce the mathematics in art and fractals.

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IB Maths Revision

I’d strongly recommend starting your revision of topics from Y12 – certainly if you want to target a top grade in Y13.  My favourite revision site is Revision Village – which has a huge amount of great resources – questions graded by level, full video solutions, practice tests, and even exam predictions.  Standard Level students and Higher Level students have their own revision areas.  Have a look!

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IB HL Paper 3 Practice Questions (120 page pdf)

IB HL Paper 3 Practice Questions 

Seventeen full investigation questions – each one designed to last around 1 hour, and totaling around 40 pages and 600 marks worth of content.  There is also a fully typed up mark scheme.  Together this is around 120 pages of content.

Available to download here.

IB Maths Exploration Guide

IB Maths Exploration Guide

A comprehensive 63 page pdf guide to help you get excellent marks on your maths investigation. Includes:

  1. Investigation essentials,
  2. Marking criteria guidance,
  3. 70 hand picked interesting topics
  4. Useful websites for use in the exploration,
  5. A student checklist for top marks
  6. Avoiding common student mistakes
  7. A selection of detailed exploration ideas
  8. Advice on using Geogebra, Desmos and Tracker.

Available to download here.

Modelling Guide

IB Exploration Modelling Guide 

A 50 page pdf guide full of advice to help with modelling explorations – focusing in on non-calculator methods in order to show good understanding.

Modelling Guide includes:

Linear regression and log linearization, quadratic regression and cubic regression, exponential and trigonometric regression, comprehensive technology guide for using Desmos and Tracker.

Available to download here.

Statistics Guide

IB Exploration Statistics Guide

A 55 page pdf guide full of advice to help with modelling explorations – focusing in on non-calculator methods in order to show good understanding.

Statistics Guide includes: Pearson’s Product investigation, Chi Squared investigation, Binomial distribution investigation, t-test investigation, sampling techniques, normal distribution investigation and how to effectively use Desmos to represent data.

Available to download here.

IB Revision Notes

IB Revision Notes

Full revision notes for SL Analysis (60 pages), HL Analysis (112 pages) and SL Applications (53 pages).  Beautifully written by an experienced IB Mathematics teacher, and of an exceptionally high quality.  Fully updated for the new syllabus.  A must for all Analysis and Applications students!

Available to download here.

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