Introduction

Sudoku originally called Number Place,[1] is a logic-based,[2][3] combinatorial[4] number-placement puzzle. The objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 sub-grids that compose the grid (also called "boxes", "blocks", "regions", or "sub-squares") contains all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which for a well-posed puzzle has a unique solution.

Completed puzzles are always a type of Latin square with an additional constraint on the contents of individual regions. For example, the same single integer may not appear twice in the same row, column or in any of the nine 3×3 subregions of the 9x9 playing board.

Sudoku Guide

Solving Sudoku is an Art. And you need some logical steps and strategies to tackle different types of puzzles from the basic one, to the fiendish. The rules for solving standard (classic) sudoku puzzles are deliciously simple. Starting from a grid containing some given numbers (seeds) the task is to fill the empty spaces with numbers so that every row, column and 3x3 subgrid (region) contain the digits 1-9. No arithmetic is involved; the process is based entirely on logic and no guesswork. Ultimately, you place a number in a square because either a) it is the only place it can go, or b) no other digit can go there.

The pleasure of solving sudoku puzzles comes from the different types of logic required. Some are more advanced than others although - even for the most difficult puzzles - much of the challenge is derived from careful observation of the grid. Elimination of candidates from a particular square or squares requires logic concepts which are actually rather straightforward; it's all down to spotting the clues.

Before we begin, a quick note on terminology:

1.Seed

A seed is any digit which appears in the original puzzle. The seeds are the starting point. Creators of hand-made sudoku puzzles often set themselves interesting challenges when providing these seeds. This can take the form of interesting, attractive patterns or - perhaps more commonly - providing the fewest possible seeds. An average (especially computer-generated) sudoku will have 26-30 seeds. Many hand-made puzzle designers won't be satisfied until they have reduced this to 24, and the majority look for less. A designer will generally be very happy with 20 seeds, which is surprisingly difficult to achieve. Symmetrical puzzles with 19 seeds or fewer are notoriously tough to make. 17 seeds is almost regarded as a "jackpot" achievement, and although we have found many examples none are symmetrical and, oddly, it appears that the majority of these are actually based on one particular arrangement of numbers in the finished sudoku. As for 16 seeds - at the moment, this seems to exist as rumour only.

2.Number

This is simply a number which the solver has committed to the grid as the only possible candidate for a particular square. For our purposes, as we describe the various types of logic needed to complete sudoku puzzles, we will use the word "number" to identify any digit, whether it is one of the puzzle's original seeds or one placed by the solver.
Introduction

Sudoku originally called Number Place,[1] is a logic-based,[2][3] combinatorial[4] number-placement puzzle. The objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 sub-grids that compose the grid (also called "boxes", "blocks", "regions", or "sub-squares") contains all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which for a well-posed puzzle has a unique solution.

Completed puzzles are always a type of Latin square with an additional constraint on the contents of individual regions. For example, the same single integer may not appear twice in the same row, column or in any of the nine 3×3 subregions of the 9x9 playing board.

Sudoku Guide

Solving Sudoku is an Art. And you need some logical steps and strategies to tackle different types of puzzles from the basic one, to the fiendish. The rules for solving standard (classic) sudoku puzzles are deliciously simple. Starting from a grid containing some given numbers (seeds) the task is to fill the empty spaces with numbers so that every row, column and 3x3 subgrid (region) contain the digits 1-9. No arithmetic is involved; the process is based entirely on logic and no guesswork. Ultimately, you place a number in a square because either a) it is the only place it can go, or b) no other digit can go there.

The pleasure of solving sudoku puzzles comes from the different types of logic required. Some are more advanced than others although - even for the most difficult puzzles - much of the challenge is derived from careful observation of the grid. Elimination of candidates from a particular square or squares requires logic concepts which are actually rather straightforward; it's all down to spotting the clues.

Before we begin, a quick note on terminology:

1.Seed

A seed is any digit which appears in the original puzzle. The seeds are the starting point. Creators of hand-made sudoku puzzles often set themselves interesting challenges when providing these seeds. This can take the form of interesting, attractive patterns or - perhaps more commonly - providing the fewest possible seeds. An average (especially computer-generated) sudoku will have 26-30 seeds. Many hand-made puzzle designers won't be satisfied until they have reduced this to 24, and the majority look for less. A designer will generally be very happy with 20 seeds, which is surprisingly difficult to achieve. Symmetrical puzzles with 19 seeds or fewer are notoriously tough to make. 17 seeds is almost regarded as a "jackpot" achievement, and although we have found many examples none are symmetrical and, oddly, it appears that the majority of these are actually based on one particular arrangement of numbers in the finished sudoku. As for 16 seeds - at the moment, this seems to exist as rumour only.

2.Number

This is simply a number which the solver has committed to the grid as the only possible candidate for a particular square. For our purposes, as we describe the various types of logic needed to complete sudoku puzzles, we will use the word "number" to identify any digit, whether it is one of the puzzle's original seeds or one placed by the solver.

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