## Must be logic only

Post the puzzle or solving technique that's causing you trouble and someone will help

### Must be logic only

There is quite a difference of opinion in how Sudoku puzzles are solved.

I belong to a group of seven people who try to solve the hardest of the puzzles that are printed in different newspapers on Sundays. We started doing this because when corresponding with different newspapers about many Sudoku puzzles that we found unsolvable we always got the same response that the puzzles were checked for accuracy by computer and were solvable by LOGIC.

In checking further we found that most of the programs used to solve these puzzles had a guessing factor programmed into them. Now as most everybody knows if the guessing factor is used then LOGIC IS NOT.

Here is a puzzle that two newspapers have printed that seem to be unsolvable logically. Although we have the answer we seven cannot solve it by LOGIC alone.

If anyone would care to try solving this and does succeed please let us know how you did it.

Thanks.

x98x12x4x
5623xxxxx
xxxxx9xxx
xxxxxx6x1
x36xxx59x
1x7xxxxxx
xxx2xxxxx
xxxxx6453
x4x57x82x
Smart3

Posts: 3
Joined: 02 October 2005

### Re: Must be logic only

Smart3 wrote:In checking further we found that most of the programs used to solve these puzzles had a guessing factor programmed into them. Now as most everybody knows if the guessing factor is used then LOGIC IS NOT.

As there are are dozens, maybe hundreds of solvers out there, I can't comment on your experience. There are plenty of solvers that are logic based, that do not use recursion or brute force (or in some cases, the user has the choice). The puzzle you have posted does not require any of this -- it is solvable by standard logical tactics. Pappocom's software (from this website) verifies this puzzle with no problem, rating it as HARD. SadMan rates it as Medium. Simple Sudoku doesn't offer ratings, but required nothing more exotic than naked triples to solve it.

Smart3 wrote:Here is a puzzle that two newspapers have printed that seem to be unsolvable logically. Although we have the answer we seven cannot solve it by LOGIC alone.

If anyone would care to try solving this and does succeed please let us know how you did it.

Why don't you post the the puzzle at the point you are stuck and will give the next step?

Code: Select all
` . 9 8 | . 1 2 | . 4 .  5 6 2 | 3 . . | . . .  . . . | . . 9 | . . . -------+-------+------ . . . | . . . | 6 . 1  . 3 6 | . . . | 5 9 .  1 . 7 | . . . | . . . -------+-------+------ . . . | 2 . . | . . .  . . . | . . 6 | 4 5 3  . 4 . | 5 7 . | 8 2 . `

This puzzle can be solved using nothing but these four tactics:

hidden singles (also known as simply "singles")
naked singles
locked candidates
naked triples

An explanation of these and other tactics is here: http://www.angusj.com/sudoku/hints.php
tso

Posts: 798
Joined: 22 June 2005

I dubbed this puzzle in to Pappocom software - verified as Hard - but completed without needing to guess anything. It is possible with logic alone. As Tso suggested, post at the point you got stuck and someone can help with the next step - after which the rest will probably fall into place.

For some reason I always find it harder to do Su Doku puzzles on paper than using the software, so I don't usually dub in unless I'm stuck. Often the next step presents itself and I wonder why I couldn't see it before
CathyW

Posts: 316
Joined: 20 June 2005

Yup, I concur with tso and CathyW. I actually found this puzzle pretty simple, and I must admit to being slightly surprised at Pappocom's attributed grade. That said, I did like this one as I don't think I've ever come across a puzzle with quite so much application of the candidates locked in a row principle, something which I need to remember to take out of my armoury more often. It's getting rusty.
Karyobin

Posts: 396
Joined: 18 June 2005

### Logic Only

There seems to be a dead end at this point.

x98612x45
5623xxxxx
xxxx59xxx
xxxxxx6x1
x36xxx59x
1x796xxxx
xxx2xxxxx
xxxxx6453
x4x57x82x
Smart3

Posts: 3
Joined: 02 October 2005

One way in:

Cells 3, 4 and 5 in row 8 (hereby referred to as r8c3, r8c4 and r8c5) form a Naked Triple, i.e. they only contain the possibilities (candidates) 1, 8 and 9, so 1, 8 and 9 must be in those cells. Removing these three numbers as candidates from the rest of that row therefore leaves a Naked Pair {2,7} in the first two cells..

This logic will take you basically to the point of completing the puzzle, just keep looking and thinking.

I'd also like to echo tso in recommending you read angusj's site. There's a lot you're missing.
Karyobin

Posts: 396
Joined: 18 June 2005

### Logic Only

Karyobin writes

Cells 3, 4 and 5 in row 8 (hereby referred to as r8c3, r8c4 and r8c5) form a Naked Triple, i.e. they only contain the possibilities (candidates) 1, 8 and 9, so 1, 8 and 9 must be in those cells. Removing these three numbers as candidates from the rest of that row therefore leaves a Naked Pair {2,7} in the first two cells..

Although the puzzle can be solved this way this is not logic, ie: Logic states that 1 and 8 can be in also be considered in row E C4 and 8 and 9 can also be considered in row G C5 This is true logic.
Using this for as naked triples is just a guess. This is what us seven are trying to state. When we wrote to a syndicated producer they agreed that this is true.
Smart3

Posts: 3
Joined: 02 October 2005

### Re: Logic Only

Smart3 wrote:Karyobin writes

Cells 3, 4 and 5 in row 8 (hereby referred to as r8c3, r8c4 and r8c5) form a Naked Triple, i.e. they only contain the possibilities (candidates) 1, 8 and 9, so 1, 8 and 9 must be in those cells. Removing these three numbers as candidates from the rest of that row therefore leaves a Naked Pair {2,7} in the first two cells..

Although the puzzle can be solved this way this is not logic, ie: Logic states that 1 and 8 can be in also be considered in row E C4 and 8 and 9 can also be considered in row G C5 This is true logic.
Using this for as naked triples is just a guess. This is what us seven are trying to state. When we wrote to a syndicated producer they agreed that this is true.

I'd be more than interested to hear an explanation - of that which I highlighted - of how this is guessing, if you'd be so kind. (no, I'm not being sarcastic, I really do want to know)

Luna
lunababy_moonchild

Posts: 659
Joined: 23 March 2005

OK - try turning it around. Neither 2 nor 7 can go in r8c3,4,5. Therefore 2 and 7 must logically form a naked pair in r8c1,2 leaving 189 as naked triple in r8c3,4,5. There is no guessing involved at all. Also, 8 cannot go in r8c3 so you can remove other candidate 8s from r7c5,6

I rarely put all candidates in to complete Sudoku puzzles - can't see the wood for the trees sometimes - but I did on this just to check that it was still obvious - and it is
CathyW

Posts: 316
Joined: 20 June 2005

### Re: Logic Only

Smart3 wrote:There seems to be a dead end at this point

Code: Select all
` . 9 8 | 6 1 2 | . 4 5  5 6 2 | 3 . . | . . .  . . . | . 5 9 | . . . -------+-------+------ . . . | . . . | 6 . 1  . 3 6 | . . . | 5 9 .  1 . 7 | 9 6 . | . . . -------+-------+------ . . . | 2 . . | . . .  . . . | . . 6 | 4 5 3  . 4 . | 5 7 . | 8 2 . `

the 4 for r2 must be in box TM [ Top-Middle ],
preventing 4 at r3c4;

so, the 4 for c4 must be in box MM [ Middle-Middle ],
preventing 4 at r6c6;

so, the 4 for r6 must be at c9.

- Pat

Pat

Posts: 3521
Joined: 18 July 2005

Gonna do a Luna now... ;-)

Smart3 wrote:Although the puzzle can be solved this way this is not logic,

What (in recent weeks) is causing this flood of rubbish? YES IT IS!

Smart3 wrote:ie: Logic states that 1 and 8 can be in also be considered in row E C4 and 8 and 9 can also be considered in row G C5

Amongst many other candidates, yes. But they don't offer any information about solving the puzzle.

Smart3 wrote:This is true logic.

'True' logic? What on earth are you talking about? Are you saying that every computing and mathematics student/graduate who frequents this site (not to mention all the highly experienced experts who have been solving puzzles for years) has not used logic when solving their sudoku? By extension are you suggesting that every technique which is built on such ground as the Naked Triple is in some way flawed? Are you saying that all our solutions are stumbled upon through incorrect reasoning? If so, you definitely win my prize for 'Most arrogant post of the week'.

Smart3 wrote:Using this for as naked triples is just a guess.

Guess? You clearly have no idea what a Naked Triple is, or what it implies in the context of a puzzle. This particular Naked Triple tells you exactly where the values 1, 8 and 9 can be in that particular row, no more, no less. This knowledge can then be used to remove candidates from other cells in that row.

Smart3 wrote:This is what us seven are trying to state. When we wrote to a syndicated producer they agreed that this is true.

Your 'syndicated producer' knows as little about solving sudoku as you seem to. Now, I/we've tried to put this politely and subtley but it's just not sinking in, is it? For God's sake, take the advice given and READ MORE! For the last time:

http://www.angusj.com/sudoku/hints.php
Karyobin

Posts: 396
Joined: 18 June 2005

Tough day in the classroom? (cheesy grin)

PS : Best bumper sticker I saw today - 'I'm diagonally parked in a parallel universe' - Isn't that just perfect?
emm

Posts: 987
Joined: 02 July 2005

I'll tell you the one I like:

'I whisper but my horse doesn't listen.'
Karyobin

Posts: 396
Joined: 18 June 2005

I haven't got a horse.
emm

Posts: 987
Joined: 02 July 2005

Oh. Doesn't seem to apply to you then.

Quick! Get back on topic!!
Karyobin

Posts: 396
Joined: 18 June 2005

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