Done, after an incredibly annoying number of guesses.

Not done, after an incredibly annoying number of guesses. Will try again after work.

start

done, 2 guesses

too tired

done... this pz is worthy of its expert rating

Hardest puzzle in the history of this site...could not work it. totally cheated. WOW.

in the HISTORY of the SITE... whoa now i feel scared

lucked out on guesses. Didn't have to start over.

well it did not feel like the hardest ever to me. true, i had to use three forcing chains - but it was easy to choose starting points for those forcing chains and they actually got you somewhere. it is much more frustrating when you try forcing and you can't even draw any conclusion from it and you have to try to pick another one and you pick several before you find one that tells you anythign.

Finally done.

Greens, no guesses. I don't know why, I'm not that good at sudoku, but it didn't feel all that difficult. I think I just got lucky early on.

That said: Can someone explain the difference between a "forcing chain" and a "guess"?

Don't they both involve "well, let's assume that this square is X number, and see where it goes from there"?

Samantha, Here is a nice write up on forcing chains. The difference, to me, is that a forcing chain reveals if a particular cell cannot have a value while a guess either puts a value in a cell and hopes for the best.

https://www.sudokuoftheday.com/techniques/for\ncing-chains/

Amply warned, I'll use pen and paper to record my guess tree. I should get through it eventually.

Rats, my first guess was really productive. Only 12 cells left to fill. Unfortunately, it was a bad guess.

go

Even though I wrote my guess tree down, I've had at least eight bad guesses, and now I'm feeling confused and headachy. Guess I'll take a rest.

Thanks for the website, KnightTime! Very helpful!!

Finally, I started in a different place and it all went well, and reasonably quickly (20 minutes). It gave me the cingratulations screen and told me I have 5 points and then reverted to my last checkpoint. Anybody would think it was Friday the 13th....

Done.

DID IT. No green. Tons of guesses. But starting again, I found it possible to do it with one bad guess and two lucky guesses.

I have an idea, that when you know you'll need multiple guesses, rather than explore all the tips of a guess tree systematically, it might be better, when stumped, to go back to the first guess. And since a productive guess is very useful, make your guess in a cell that is the intersection of productive looking chains, even if you are not making a binary guess. Just a thought.

go

finally!! this one really kicked my butt.

go