need unsolvable sudiku

flextone · Posted: Mon Feb 06, 2006 3:58 pm Post subject: need unsolvable sudiku

I've been playing with Simple Sudoku's Extreme level puzzles and have yet to find a puzzle which is not solvable with a combination of:

* the most basic techniques (naked and hidden singles and pairs, and what Simple Sudoku calles Locked Candidate Exclusion);

* swordfish;

* single-number multiple-chain coloring (especially creating a loop consisting of weakly connected (via exclusion) chains of even order, and at most one chain of odd order, where a chain consists of conjugate pairs).

I find the latter easier to spot even than naked triples (a fortiori hidden triples or quads) or xy-wing since it only requires consideration of one number at a time.

* Maybe an occasional naked triple (I'm not sure if this was required; I know that I've spotted it by accident at times and used it, don't remember if I backtracked and tried to do the puzzle without it.)

Can you masters of the databases pull up any puzzles which require any techniques other than the above-named one?

Thanks!
flextone
reefclub

evert · Posted: Mon Feb 06, 2006 4:06 pm Post subject:

Probably only T&E:

Carcul · Posted: Mon Feb 06, 2006 5:54 pm Post subject:

Henk · Posted: Mon Feb 06, 2006 5:59 pm Post subject:

This is my collection. Any puzzle meets your requirements, but are still solvable with logic, you just need some more advanced techniques.

Puzzle collection
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Henk · Posted: Mon Feb 06, 2006 8:13 pm Post subject:

Even better, try this one! Just generated these for you... Razz

Pretty hard..
Even harder..
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Ruud · Site Admin

Those were all pretty tough. Now try this one, flextone.

Henk · Posted: Mon Feb 06, 2006 8:50 pm Post subject:

Ruud:
Nice one, i'm impressed! My solver started to sweat because of your puzzle, couldn't solve it without guessing! Generated yourself?
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Ruud · Site Admin

tarek · Posted: Tue Feb 07, 2006 12:32 am Post subject:

I liked the Nightmare puzzle on your site Ruud, too many fishes. Very Happy

chuckfresno · Posted: Tue Feb 07, 2006 2:37 am Post subject:

There are six here that are beyond these tactics -- at least 4 are beyond simple forcing chains and/or nishio.

SOLO, available free as part of Simon Tatham's Portable Puzzle Collection, set on UNREASONABLE will give you a steady supply of very hard puzzles, most of which will require comprehensive forcing chains or forcing chains with mutli-cell links or other creative methods.

The free SUDOKU SUSSER will fetch puzzles from the MENNESKE site which has a wide variety of both difficulty and variants.

Sudoku Cue typically requires either forcing chains -- sometimes simple (bivalue) sometimes more complex.

Another good site for harder puzzles is here complete with very specific rating system.

The Super Tough at KRAZYDAD also fit the bill.

foxglove · Posted: Tue Feb 07, 2006 11:40 am Post subject: May it serve you well

4 . . . 6 . . . 7
. . . 4 1 2 . . .
. . 6 . . . 4 . .
. 6 . . . . . 1 .
1 7 . . 2 . . 5 8
. 5 . . . . . 4 .
. . 2 . . . 9 . .
. . . 2 3 8 . . .
5 . . . 7 . . . 3

Ruud · Site Admin

Nice diamond, foxglove! Cool

Own production? Source?

This one's even more polished (and difficult)

foxglove · Posted: Tue Feb 07, 2006 9:50 pm Post subject: diamonds

Own production, but sheer luck it just came out as I was reading the post.

Yours is certainly more polished and much more brutal as hardness goes:

mine is more suffer till dawn:

Miles · Posted: Tue Feb 07, 2006 10:27 pm Post subject:

What are these graphs ?

foxglove · Posted: Tue Feb 07, 2006 11:34 pm Post subject: