dailysudoku.com

[immpy]

Happy New Year to the community, enjoy the puzzle...

[ZeroAssoluto]

Hi everyone,

after basic

[Marty R.]

I hate to give up, but I'm giving up. I spent more time on this than a sane person should. I couldn't find a pattern-based solution and couldn't put together an AIC, which I'm especially bad at.

[arkietech]

[Ajò Dimonios]

Hi everyone

This sudoku has 25”backdoors” that directly solves the puzzle with the basic technique but does not demonstrate the uniqueness of the solution: R1C3=8, R1C5=7, R1C7=2, R2C4=5, R2C5=1, R2C6=4, R2C7=6, R2C9=8, R3C2=7, R3C4=2,R3C5=9, R3C6=8, R3C7=1, R4C2=8, R5C3=4, R6C7=8, , R7C2=4, R7C4=9, R7C5=5, R7C6=7, R7C7=3, , R8C4=4, R9C3=7, R9C5=2, R9C7=4.
To demonstrate the uniqueness of the solution:
R2C9=6=>contradiction (R3C2=Ø)=>-6R2C9=>Stte.

Ciao a Tutti
Paolo

[immpy]

This puzzle was (and still is) full of intrigue. On my first attempt at solving it, I just kept weeding and rinsing...and I managed to solve it with BASICS ONLY! I amazed myself (to say the least), because the puzzle had a high difficulty rating of "unfair" according to the software I used to create it. I never noted any of my advanced move techniques, which I do as I solve harder puzzles, because there were none to note! And I know that puzzles can vary....being most difficult at some lower rating, or somewhat easy at higher difficulties....I have seen it many times. I am assuming this depends on what comes easy or hard for the human trying to solve a puzzle. Each of us have an ability to see some things better or worse than others see these same things. Was I beginning to "see" patterns with more proficiency and adding them into my scans without noting them?? It's very possible, but I'm not sure of this.
So this explains why I posted it as a "Top Shelf Hard" puzzle. I assumed that basics was all that was needed here.
Now for the intrigue.....Marty R. posted his reply....I was puzzled now. So I ran the puzzle through Helmut's website, and it went only so far before it said "Sorry, this puzzle is beyond this humble computer's solving powers." I could not believe it as I read the words.
So I reloaded it into the program that created it. It is called HoDoKu. The program went through each step, while giving hints, that would be used to solve the puzzle. Besides basics it found X-Wings, XY-Wings (at least one flightless), a Nice Loop, a discontinuous Nice Loop...I think that was about it with singles remaining the rest of the way. Folks, I suck at Loops, so I sure didn't use those techniques. How did I solve this puzzle????? This question still has me wondering (and probably always will). I followed each step via the program until the conclusion....and quite happily the end result was that MY solved puzzle matched the program's solved puzzle. WAHOO!!! And the program would have warned me if this puzzle was not "Unique" (having multiple solutions). This warning comes immediately upon loading or at any step in the process of adding or removing numbers from a puzzle.

As a side note, my pencil and paper puzzle had NO pencil marks left in it...ALL blocks were filled with singles. On the harder puzzles I leave the pencil marks in each square with my ending value in a circle so I can go back if I find a contradiction or make a mistake (of which, I make my share indeed, now in my later years). LOL

And finally Ajo Dimonios' reply referencing "backdoors". What are these?? And what do they mean when it comes to solving a puzzle??? Ajo said he found 25 of them.

If anyone can explain any of this to me, it would be greatly appreciated.

I apologize for the length of this post, but this puzzle surely will go down as the ONE I will always remember, whether I never do another one, or whether I do thousands more in my lifetime......

Thank You all.......

immpy