dailysudoku.com

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

I came across a 2006 article in USA Today referring to a Finnish mathematician who claimed he had created the hardest Sudoku ever (named Al Escargot - the puzzle not the Finn). I downloaded it from this blog site:
http://photos1.blogger.com/photoInclude/blogger/3235/1499/1600/Al%20escargot.jpg

I started it but it's brutal - after a few moves I got totally stuck - though I will persevere. I hope it's a genuinely solvable and one solution problem.

Try it out. But be afraid !!

Marty R. · Posted: Thu Aug 23, 2007 10:33 pm Post subject:

Well Craig, a few moves is more than I could do. I'll be the first to embarrass myself and say that after writing in the pencil marks I couldn't eliminate one candidate. Embarassed

Asellus · Posted: Fri Aug 24, 2007 4:49 am Post subject:

There is a finned swordfish on 9 that eliminates four <9>s in B8. After that, I am, so far, stuck.

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

I looked at all the popular techniques discussed on this site and couldn't get past a few preliminary steps (though there might be some advanced techniques I've heard mentioned e.g. Sue Coq). Anyway I did finally solve the thing by guesswork - and yes, there is a unique solution.

But I had hoped that all puzzles were solvable by existing techniques, without guesswork. Maybe they still are.

Ruud · Joined: 18 Jan 2006 Posts: 31

There's more info on this puzzle at the Player's Forum.

It is no longer considered the hardest Sudoku ever. Here's a list of the hardest puzzles. Don't try these at home.

Johan · Joined: 25 Jun 2007 Posts: 206 Location: Bornem Belgium

cg,

If this puzzle has a unique solution there must be some logical solving technique,(probably somewhere hidden and yet to be discovered?), for cracking this tough nut(which is an understatement).

In my opinion anyone who can solve this one with only logical solving techniques, has find the ultimate solution path for every SUDOKU, even the hardest one ever designed, like this one.
Chapeau for the one who can solve this without T&E.

Johan

keith · Posted: Sun Aug 26, 2007 2:59 am Post subject:

Johan,

You may want to look at the Sudoku Programmer's Forum:

http://www.setbb.com/phpbb/?mforum=sudoku

As I recall, you can determine that a particular puzzle does / does not have a unique solution without "solving" it in human terms.

As I understand it, there are Sudokus which cannot be solved other than by trial-and-error. The nice term for this is "exhaustive search": Try every possibility.

At the risk of irritating the computer scientists who read this: Exhaustive search is how you find the next Mersenne Prime, or prove the four-colour map theorem.

Or, for readers of this forum: The only way to prove a puzzle is valid (has a unique solution) is to solve your puzzle using only methods blessed by Glassman.

Best wishes,

Keith

Mogulmeister · Joined: 03 May 2007 Posts: 1151

Indeed - and most computer solvers make use of this algorithm although you need the brute force capabilities of a computer to do it.

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

Mogulmeister · Joined: 03 May 2007 Posts: 1151

If you look in the Sudoku programmer's forums you will see some conversation with Arto Inkala himself the creator of AI Escargot.

Swampy · Joined: 27 Mar 2008 Posts: 1

I found this puzzle about 15min's ago

Have finished it and can confirm a logical solution

ravel · Joined: 21 Apr 2006 Posts: 536

The solutions i have seen so far for puzzles of this difficulty took me more than an hour to read and verify them (and it was more work than fun).

So what did you do in these 15 minutes ?

nataraj · Posted: Fri Mar 28, 2008 9:44 pm Post subject: Re: Completed puzzle

waleeed12 · Joined: 26 Mar 2016 Posts: 2

this site and couldn't get past a few preliminary steps (though there might be some advanced techniques I've heard mentioned e.g. Sue Coq). Anyway I did finally solve the thing by guesswork - and yes, there is a unique solution.

waleeed

dongrave · Joined: 06 Mar 2014 Posts: 568

The link to the hardest Sudoku puzzle ever seems to be broken for me but luckily Gordon mentioned it by name so I was able to find it. I'm guessing that this is the puzzle that everyone is referring to here.

dongrave · Joined: 06 Mar 2014 Posts: 568

Of course I was being sarcastic in my last post. I wasn't able to make ANY progress at all after the basics so even though I hate to, I'm going to have to use a computer to tell me if this thing can be solved using only moves that are commonly used on this site (i.e. basics, wings, fish moves, chains, etc.). And while I'm at it, Keith has inspired me to find the 50th Mersenne prime! (I expect it to only contain somewhere around 30 million digits.)

dongrave · Joined: 06 Mar 2014 Posts: 568

Well, apparently according to the computer it's possible to begin solving this puzzle by showing that r2c6=9. Now I'm not saying that it's easy - but at least it appears to be possible! I didn't check it for correctness (since that would take me the rest of my life) but it was able to show after a couple dozen advanced chains that r2c6 could not be 1 or 5 so it must be 9.

Then after it determined that r2c6=9, it's apparently possible (after another dozen chains) to eliminate the 4 from r5c3 so it must be 3. And then another dozen chains later it showed that r2c1=5. THEN another dozen chains later it showed that r6c3=8 and after that, it ONLY required a few chains to prove that r7c3=2. It was only at this point that it was finally able to solve it - but NOT by using just basics; it STILL required more advanced chains! This puzzle is absolutely insane!!!

Strangely enough, this was the ONLY solution that it found (with my help) using the common moves used on this site - and there were a number of times that it could have used an advanced move other than a chain (like the swordfish that Asellus mentioned and several wings) but I had it focus on chains. If anyone finds anything else (either with or without the use of a computer), I'd love to hear about it! Thanks, Don.

keith · Posted: Sat Apr 02, 2016 7:56 am Post subject:

Zombie thread!

I believe there are puzzles much more difficult for a human solver than this one.

I may still have the poster from Cray Research in my attic that has the 29th Mersenne Prime. That filled a 2x4 foot poster with digits in a fairly small font.

Keith

dongrave · Joined: 06 Mar 2014 Posts: 568

What's it doing in your attic? Why isn't it on your wall? You know Keith - it'll probably take you close to 1000 posters that size to print out the next one when they find it!

Hey! By the way, when I searched the internet looking for 'Al Escargot', I saw that the guy that created it followed it up with a new one that he says is much more difficult - so now apparently they're referring to this one as the 'hardest Sudoku in the world'. Here it is:

waleeed12 · Joined: 26 Mar 2016 Posts: 2

There is a finned swordfish on 9 that eliminates four <9>s in B8. After that, I am, so far, stuck.

== Solitaire ==