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  Strategy Families
There are two ways one can group all the stratgies for Sudoku. By difficulty and by family. Difficulty is rather subjective but necessary, for example, when selecting which strategies to test for in the solver. Some strategies will always be easier for some people to spot than others but I believe I have chosen an ordering which is not too controversial. So the main documentation has a side menu organised by difficulty.


Basic Strategies Chaining Strategies Exotic Strategies Uniqueness Strategies

'Bent' Sets
Articles: Strategies: Forcing Chains sub group:
Sudoku X Strategies
Jigsaw Strategies Killer Strategies

With chaining strategies there is definitely a theme going through them. This theme is all about bi-value (two candidates left in the same cell) and bi-location (two candiates left in the same unit) pairs and the incredible number of deductions one can make from them. You will find, if you read through this group, that earlier strategies become part of a more general theory as the theme develops. Thus for example, Remote Pairs are a sub-set of XY-Chains; that is XY-Chains is a more general approach of which Remote Pairs are specific instance. Do read the introductory articles Introducing Chains and Links and Weak and Strong Links.

Exotic strategies do overlap with chaining ones, but they have a peculiar flavour of their own and some wonderful, if obscure, logic. They are definitely worth presenting as a demonstration of peoples ingenuity but you will only need to have recourse to them on the extreme puzzles.

There are naturally special strategies for Jigsaw and Killers because of their differences. These are now included for the first time on this site.

This strategy list is by no means complete. Many can be further extended and we do not have a complete theory of all Sudoku puzzles. If you are interested in the concepts behind creation and grading there is a PDF document here called Sudoku Creation and Grading. With the communities help I hope to extend the documentation here.




For those people wondering why "Escargot" cannot be solved by the solver, there is an article on this special Sudoku here. This is an early 'untimate puzzle' but this crown has been usurped by the puzzle created by Arto Inkala, also in the example list.

Articles
Article: The Relative Incidence of Sudoku Strategies

Article: The 17 Clue 'Proof' - my take on the January 2012 computerised exhaustive search proof.

Article: The Chaos Within Sudoku - A Richter Scale on a paper by Maria Ercsey-Ravasz and Zoltan Toroczkai at the Faculty of Physics, Babes-Bolyai University, Romania. Very interresting paper.

Article: Generalizing Sudoku Strategies by Kevin Gromley, March 2014

My response to Crooks Algorithm is here.



I'm pleased to include on this web site the Sudoku Song (MP3 file) by Peter Levy (official web site here). Peter wrote and recorded this song a couple of years ago and managed to capture the essence of the Sudoku craze to great aclaim.





 
   
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Comments - Talk

Thursday 20-Jun-2013

... by: P Majumdar, Delhi, India.

I fully agree with the Comments from Mae (Thursday 9-Frb-2012.)

All easy, medium and other low-difficulty level tough Sudoku puzzles cover only the very few basic simple strategies, whereas, tough/diabolic puzzles cover the remaining galaxy of advanced stategies. It is a pity!

The creator/s of all tough/diabolic Sudoku puzzles, should, provide some minimum hint, at least by naming the strategies involved towards solving the puzzle.



Saturday 23-Jun-2012

... by: jennifer antonio

cool

Thursday 9-Feb-2012

... by: Mae

The so called easy Sudoku puzzles on Road Runner have just gotten too hard to make them fun any more.
Sometimes it is fun to just take a few minute break and work and easy Sudoku. How can you encourage people to begin playing Sudoku, if you make them this tough?


Wednesday 18-Jan-2012

... by: moses

I am making my own booklet on sudoku strategies may i use your samples from your book?

Andrew Stuart writes:

No

Wednesday 10-Nov-2010

... by: Jonathan Carr-Hopkins

If, as Geoff in Australia offers, one cell can be only 3 or 5, then selecting one, and if that doesn't work, using the other, is this process, known as iteration, 'cheating', or is only a formulaic solution the only bona fide and acceptable one?
Jonathan


Sunday 13-Jun-2010

... by: Geoff

I have been doing the Sudokus out of the local paper (Queensland Times, Australia) and they are rated from One Star to Five Star.

I have just run across the first Five Star Sudoku where I am 3/4 finished but can not find any more hints.

Some of my friends tell me that you now need to start a 'trial & error' process ie if one cell can only be a 3 or a 5 - then pick on the 3 and see if it carries all the way through and if not, go back and pick on the 5.

I tend to think that there should always be a 'hint' to be found - is this correct?

Geoff


Tuesday 18-May-2010

... by: Lloyd Welton

Thanks for the great book. Never seen so many strategies so well explained in one book. Maybe in any following editions it should be spiral bound, that way it would a bit easier to use it as a reference.

Thanks again.


Thursday 20-Aug-2009

... by: John C Raaen, Jr.

I blew it! The setup for a Virtual Wall is A1=1, A3=3, D2=4 and B4=4. Then A7, A9, C1, C2 and C3 are the "candidate squares for "4". Sorry!

Wednesday 19-Aug-2009

... by: John C. Raaen, Jr

I see no examples of my concept of a wall or virtual wall. If boxes are A thru I and the squares are 1 thru 9, fill A1=1, A2=2, A3=3. This is a wall. Any number other than 1, 2 or 3 in box B or C, say B1=4. can be placed only on squares C1, C2 or C3, and on A7, A8 or A9. Usually some of those candidate squares are already occupied, often leading to an easy placement of, in this case, a "4". (Of course, the squares B and C, 1, 2 and 3 are excluded in the above.)

In a virtual wall, use the same setup but leave A2 blank and put the "2" on D2=2. Again, the "4" can be placed only on squares C1, C2 or C3 and on A7, A8 and A9.

I am sure this simple technique is included somewhere on this site.


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