Solver App for Android
Strategies for Popular Number Puzzles
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Feedback and Questions

I've received a lot of interesting comments and questions from Sudoku fans over the last few years and this page is where I try to answer them. I'm also directing Str8ts feedback here. Please feel free to drop me a note on the side of the page. Or you can email me directly at .

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Many thanks to all the people who have helped improve the solvers and strategies with their feedback!

Saturday 29-Mar-2008

... by: Mal, BC Canada

Load Sudoku:

Hi this is a great web site, the best I have found. This puzzle locks up when it gets to "X wing" and the comments box says SERVER ERROR.
I have cleared the browser and re booted over a 3 day period but still get the same error. Is this problem my end or your end.

Andrew Stuart writes:
We have had some technical problems over the last few days, as several of you reported. The problems should now be corrected. I tried your sudoku and it solved for me.

Sunday 17-Feb-2008

... by: Booth, Kailua, Hawaii (Oahu)

I was wondering about the singles chain. I understand why it works but whenever I come to the solver and get to the singles chain I never see why you're eliminating numbers. At the bottom it says type 1 or type 2 singles chain. Does that mean I should be using type 1/2 multi coloring? I can usually figure it out by saying "if this is this then that is that and so forth" and I find a contradiction. But I'm trying to find a simple way of seeing it and I just can't from your solvers point of view. I know I'm missing something but I don't see it. BTW, awesome site! It's marked as one of my favorites!

Andrew Stuart writes:
I should perhaps beef up the documentation on that.

Basically Type 1 is where eliminations are made off the chain - ie there is a clear set of candidates with alternating colours but no contradition. Two cells with different colours point to a third candidate off the chain which can be removed since one of those two colours must be a solution

Type 2 occurs on the chain. That is where two of the same colour appear in the same unit. All of those of that colour must be wrong.

Hope that helps a bit

Monday 11-Feb-2008

... by: Dawn, Littleton, Colorado USA

Just to let you know your solver rocks.

Andrew Stuart writes:

Friday 25-Jan-2008

... by: Bert (Hopeless), Cleveland, Ohio

Load Sudoku:

Andrew, first off - my commendations on your solver, which I use regularly to see things I might have missed in solving a tough puzzle. I believe there is a problem with your "empty rectangle" strategy, however. The above puzzle is Sudocue Daily Nightmare from September 4, 2007. After the initial basic removals, there are empty rectangle patterns for both digit 4 and digit 6 in r3c1/r3c8 using box 7 as the hinge and removing both 4 and 6 from r7c8. However, the solver goes right past this into ALS removals. Thanks.

Andrew Stuart writes:
Extremely well spotted. Both your example and my bug. I am testing an expanded version of my ER strat at the moment, just in case it makes incorrect eliminations. So far so good. Basically I was too cautious about the circumstances that should be looked out for.

New version of the solver loaded. Captures this example.

Wednesday 23-Jan-2008

... by: Drue, UK

You have saved me from mental torment....!!! Don't know if anyone else was stumped by difficult puzzle in The London Paper 11 January, but I've been tussling with it for days (and nights). Even wrote it out LARGE ...Used your site to get me past 2 steps - quadruples in a row and then the X-wing. Thank you, thank you - now I can sleep in peace.....
Will definitely use your site to learn more techniques.

Andrew Stuart writes:
Great its working for you! Best of luck

Wednesday 2-Jan-2008

... by: jar29, perth, australia

thankyou for setting up this site. i was ready to top myself if i hadn't solved that bloody puzzle. i'm sure i will be back. have fun

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Thank-you everyone for all your questions and contributions.