Tough Sudoku

Good maen all in sudokuland

6.30

10:10

16.24 where's the stairs?

oh my god!finished it in flat eight mts.yes me to wonder wheres the stairs going

OK, I got it, but I admit I was lucky. I was looking for a forcing chain and the whole thing broke on me.

Hi Dan from Oz,
You did it quite quick. Any system except setting up all possibilities and looking for figures with one option only?
Gaby

Partial solution:
1. a3=b1=24 elim in box, easy to 27 filled
2. h1=h4=24 elim in col, easy to 54 filled
3. d8=e9=f9=469 elim in box
4. X wing on 9s d1,e1,d6,e6 => f9=9

so, at 55 filled my grid looks like this:
5 7 3 1 4/6/9 4/6/9 2 8 4/5/6/9
1 8 2/4 4/6/9 2/7 5 4/7/9 3 4/6/7/9
9 6 2/4 8 3 2/7 4/5/7 1 4/5/7
4 2 5 6/9 6/9 3 1 7 8
7 9 8 4/5 1 2/4 3 6 2/5
6 3 1 7 2/5 8 5/9 4 2/5/9
2 5 6 3 8 4/7 4/7 9 1
3 1 9 2 4/6/7 4/6/7 8 5 4/7
8 4 7 5/9 5/9 1 6 2 3

1 am, work tomorrow, so I'll dream of a forbidding chain :)

A difficult one this...
I think I'll try it later...

17.54 stuck at 27 filled...printing on paper and will try to finish. i'm still finding these really difficult to do...i can usually do 1 in 20 or 30...but then i used to find the 'hard' really difficult too, now they're always done

No timer. Great moody picture! Take care, everyone.

1) Start at 22 filled. Unique Possibilities to 27.
2) Hidden pair 69: d6=9 == e6=9 -- e6=6 == d6=6 forbids e4=9, d6=45, e6= 245.
Locked 5's forbid b4,c5=5. Locked 5's forbid b1=5.
3) The Almost Locked Set a6=247, b6=2457,c6=245,h6=247 and the wrap around chain:
b1=2 == b1=4 -- h1=4 == h1=2 forbid b4=42 ( note, b4=42 will either leave no 2's or no 4's in the ALS.)
4) UP to 56. X wing on 4's forbids e8, i78=4. i2=7 == e2=7 -- f3=7 == f7=7 forbids i7=7.
5) Up to 81
sets 14, depth 6

11:07 not bad for me
Great picture. I love the beach all year round.

As a little provocation, let me offer this 'solution' using hints from Simple Sudoku (SS):
UP to 27; SS elims;
abc6=4==h6=4--h1=4==b1=4 elim b4=4; strong sets (i3=1,i2=1),(b2=1,b2=3),(h1=2,b1=2),(b4= 3,b4=2) elim i3=2; SS to end. Depth: 4! (Too many sets, I am sure; but they can be cleaned out with more time...).

Note one can write the fc for eliminating 42 at b3 like this:
b1=2 == b1=4 -- h1=4 == h1=2
then note h6=4 == abc=4 and h6=2 == abc=2 to forbid d4=24.
Reduces depth to 3.

Yes Andrei - cross posting there - depth 3 is possible - just I sort of like to eliminate them both with one fc... but two fc's of depth 3 suffice.

I suppose if one uses pair 42 earlier, then depth 2 is possible.

Same number of sets as my solution above, I think, just depth 2. Do the pair 42, then X wing style (from gb's nomenclature)on 4's and 2's to eliminate 42 at b4.

Interestingly enough, perhaps, one can use the hidden pair 69, without doing any eliminations except locked 5's in row 1, to prove strong 4's and 2's in the hidden locked set 2457, which then with the pair 42 eliminate 24 at d4. My first proof above should have noted that I had a locked set, not an Almost Locked Set. Sorry about that.

1) Start at 22 filled. Unique Possibilities to 27.
2) Hidden pair 69: d6=9 == e6=9 -- e6=6 == d6=6 forbids e4=9, d6=45, e6= 245.
Locked 5's forbid b4,c5=5. Locked 5's forbid b1=5.
3) Pair 42 at b1, h1. abc6=4==h6=4--h1=4==b1=4 elim b4=4;
abc6=2==h6=2--h1=2==b1=2 elim b4=2
4) UP to 56. X wing on 4's forbids e8, i78=4. i2=7 == e2=7 -- f3=7 == f7=7 forbids i7=7.
5) Up to 81
sets 14, depth 2

I actually prefer my first proof, becuase of the symetry. But, latter proof is less deep.

to nic: How did you eliminate 3 from h4?

Forgot timer, but felt average (for me)

18:58 had one guess because I can't figure out Steve's comments.

7:26

all 4 done for the day, so I guess I should finish lunch and get back to work
Hope everyone is enjoying the day.

13:12

16:20 w/ couple of guesses

6:20

re your first post on tough today, what do you mean by 'sets' and 'depth' in 'sets 14, depth 6'? Forgive my ignorance, please, and thanks so much for putting your thought processes out there for all to study.

Thanks to you, too, for the same.

By 'sets', we mean the number of 'strong' sets considered. 'Strong' sets being a set in which at least one of the possibilities listed must be true. By depth, we mean the number of strong sets considered at one time to produce an elimination. Generally, at the end of a proof, only the longest chain is mentioned. (maximum depth).

Depth and sets are then some sort of objective measure of the difficulty of a proof.

Steve from Ohio - Considering the number of people who seem interested in learning....and given that terminology is always changing....it might be a good idea to consolidate some of this on a page somewhere - definitions, FAQ's. What do you think?

4.12 no system just guesses!!!

I am also a newbie and cant work out the jargon. What does abc6=2 mean?

14:51, lucky 'guess' looking for a forbidden chain...
I believe if you go to 'MORE' and 'PROOFS' that gb's nomenclature is explained there... this is the same I think as what Steve/Ohio uses. I admit to being a bit confused by it all, and just try to get through on my own anyway...