Sudoku Explained

By Giulio Zambon

This book tells you in a very practical way how to solve Sudoku puzzles.
First, it describes in detail and with examples fourteen solving strategies, from the very simple to the very complex.
Then, it explains how to apply the strategies you have learned to forty puzzles of increasing levels of difficulty.
No other book will teach you how to solve Sudoku puzzles as effectively as this one, but be warned: after reading it, you will discover that the puzzles in your local paper are not so challenging after all!

• Language: English
• Publisher: Giulio Zambon
• Release date: Nov 8, 2011
• ISBN: 9781466170612

Giulio Zambon

Giulio holds a doctorate in Physics from the University of Rome (Italy), where he graduated cum laude. After a period of teaching Electrotechnics in a high school and a fifteen-month stint in the technical corps of the army, he joined the computer industry. In the ensuing thirty years, he worked as a software developer, system consultant, process improvement manager, project manager, and chief operating officer. During his career in computing, Giulio wrote hundreds of technical reports and presented several courses and seminars. He published several computing books and manuals and some Science Fiction stories. The list of his published work is at his website zambon.com.au. In October 2011, he released CalcuDoku 1.0, an application to play CalcuDoku on the iPad. You can find it in Apple's iTunes application store.

Book preview

Sudoku Explained

By Giulio Zambon

Copyright 2011 Giulio Zambon

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Table of Contents

Introduction

Getting Started

The Strategies

Level 0

Level 1

Level 2

Level 3

Beyond Level 3 Strategies

The Puzzles

The Solutions

Level 0

Level 1

Level 2

Level 3

Completed Puzzles

About the Author

Introduction

Welcome to Sudoku Explained. The purpose of this book is to help you become a Sudoku champion. Not everyone has the pattern-recognition abilities to become a champion, and I cannot guarantee that you will succeed, but I can point you in the right direction. The rest is up to you.

The WorldWide Web is full of explanations about Sudoku. You can find the description of many strategies and lots of examples. What I believe you will only find in this book is a systematic and consistent description of all significant strategies coupled with commented solutions of puzzles.

For this book, I have chosen forty Sudokus I generated with the program described in my first Sudoku book (Sudoku Programming). They all have thirty-three clues and look like medieval crosses, because the central row and the central column are always already solved.

Their difficulties range from easy to diabolical, measured as the number and complexity of the solving strategies they require. It is true that, in general, puzzles with more clues are easier, but what is true statistically, doesn't apply to each individual puzzle. I have seen very easy puzzles with only seventeen clues, while the last puzzles in this book, despite their thirty-three clues, are as challenging as they come.

Getting Started describes the terminology I use throughout the book.

The Strategies explains in detail fifteen strategies grouped in four levels of complexity, from 0 to 3.

The Puzzles presents the Sudokus. Puzzles 1 to 10 can be solved with level 0 strategies; 11 to 20 require level 1 strategies; 21 to 30 are at level 2, and 31 to 40 are at level 3.

The Solutions provides the solutions of the puzzles shown in the preceding chapter. For the puzzles at level 1, I also list a sequence of strategies capable of solving them, while for the puzzles at level 2 and 3, I explain the application of the more complex strategies in detail.

Have fun with Sudoku!

Getting Started

In this chapter, I describe the terminology I will use in the reminder of the book to explain Sudoku solving strategies and to illustrate the examples. Although you will probably be familiar with most of it, I suggest that you still go through this chapter.

A Sudoku puzzle is a grid of 9 x 9 cells partially filled with numbers between 1 and 9, as shown in Figure 1 1.

Figure 1 1: A Sudoku puzzle

You identify each cell through its row and column indices (see Figure 1 2), and the term line indicates either a row or a column.

The nine groups of 3 x 3 cells delimited by thicker borders are called boxes. You identify them with an index between 0 and 8 as shown in Figure 1 3. The term unit indicates either a line or a box.

To solve a Sudoku puzzle you need to complete the grid with numbers between 1 and 9 in such a way that each number only