... by: Rajk, Netherlands

Escargot
Escargot does not have a solution?
Run out of known strategies.
Andrew Stuart writes (3-May-2008):
Yep. That is a semi-famous sudoku invented by a Finnish mathematician who claims it is the hardest sudoku ever. Currently aboyt 0.2% of randomly produced sudokus can't be solved with my logical strategy set so there is plenty of work still to do...

... by: Murray, Australia

Load Sudoku: 352718946819462735467359182000605420524801600673294518246987351705120804000540200

Hi Andrew, in solving the above problem I encountered the Diabolical strategy called "20 Bug". there is no explanation for this when I click on the link. Perhaps this is an accidental omission.

regards

Murray
Andrew Stuart writes (8-Apr-2008):
Please load your sudoku and click Solution Count. Your sudoku has 5 solutions. When it Takes Step, the solver does not check for multiple solutions. Due to use of strategies that depend on the sudoku having a unique solution, the solver may produce strange results for sudokus having more than one solution. Furthermore, Take Step may find a solution without giving any indication that other solutions exist.

... by: Andy Fraser, UK

Excellent solver. Hopefully I will learn to spot these patterns eventually.

I have a slight problem though:

Using the back button "<<" takes it back more than one step; sometimes even removing correct numbers I have put in manually.

I am using Firefox 2.0.0.13 in WinXP home.
Andrew Stuart writes (3-Apr-2008):
There is an issue with entering numbers and using the back button. Back really only works after you have "taken step" and it is remembering the state the board was in when you pressed that button, not when you entered something. I'd avoid that button for un-doing manually entered numbers

... by: JohnR, USA

Andrew,
Now that sudoku.org.uk seems to have died, I believe there will be a population wanting to continue a daily-weekly competition. Thoughts?
Andrew Stuart writes (3-Apr-2008):
I have started posting a daily sudoku on my site at http://www.scanraid.com/Daily_Sudoku.asp So far, I haven't set up a competition. It does look like the current administrators at sudoku.org.uk have worked out the glitches and are keeping the competitions alive. Since that site is outside my control, I won't speculate on what changes they may implement. However, the problem of experts lists that are top heavy with perfect scores, that I addressed last year, remains. The spread of scores, that we originally had, is long gone. The number of perfect scores continues to grow, resulting in ever more lengthy experts lists. How the new administrators will handle the problem remains to be seen.

... by: D. Meyer, Billings, Montana

Load Sudoku: 043056000070000543000340026000193004304605200000024300910532400038400000400968130

How come your solving page states. "run out of known strategies" for this particular board. But the newspaper prints a solution which I can send to
you if you wish?
.43.56...
.7....543
...34..26
...193..4
3.46.52..
....243..
91.5324..
.384......
4..96813.

Andrew Stuart writes (3-Apr-2008):
I clicked Solution Count and the solver found 14 solutions. The Unique Rectangles strategy is only valid for sudokus that have exactly one solution. Unique Rectangles may remove all solutions from a sudoku that has multiple solutions. Commonly, multple solutions are the result of omitting one or more givens when entering the puzzle. If you are part way through the solution process when you discover that you left out a given, it is best to Clear and re-enter the whole sudoku. The solver may have already eliminated a candidate that is part of the solution and, thus, no longer be able to find the solution.

... by: Mal, BC Canada

Load Sudoku: 312050067458007320967230540673845192801300075509710083006003254004500036235000710

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.
Thanks
Mal
Andrew Stuart writes (3-Apr-2008):
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.

... 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 (22-Feb-2008):
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

... by: Dawn, Littleton, Colorado USA

Just to let you know your solver rocks.
Andrew Stuart writes (12-Feb-2008):
;-)

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

Load Sudoku: 000648000000000000038090501520000000000000003007910050900080700072000090085000230

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 (28-Jan-2008):
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.

... 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.
Drue
Andrew Stuart writes (28-Jan-2008):
Great its working for you! Best of luck

... 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