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NoSudoku123456789 wrote:Multiple solutions?

Hint : incomplete analysis of the digits 4, 5 and especially 7.

- JC Van Hay
**Posts:**697**Joined:**22 May 2010

The puzzle solved status in line format : .8...7.1...4.1..8.1.968.2.49..87....81...9.264.....89.658..2..9.4..98.62291....38

Solves with one Skyscraper in 7's :

Skyscraper in Cells a-b-c-d => - 7 r2c1, r3c8 and the puzzle solves in singles from there (called stte for short). A link to a write-up on Skyscrapers is this.

Leren

Solves with one Skyscraper in 7's :

- Code: Select all
`*-----------------------------------------------*`

| 35 8 26 | 249 24 7 | 69 1 35 |

| 35-7 26 4 | 29 1 35 | 69 8 d357 |

| 1 a37 9 | 6 8 35 | 2 5-7 4 |

|----------------+---------------+--------------|

| 9 236 2356 | 8 7 146 | 34 45 15 |

| 8 1 357 | 345 345 9 | 347 2 6 |

| 4 b367 3567 | 235 2356 16 | 8 9 c157 |

|----------------+---------------+--------------|

| 6 5 8 | 1347 34 2 | 147 47 9 |

| 37 4 37 | 15 9 8 | 15 6 2 |

| 2 9 1 | 457 456 46 | 457 3 8 |

*-----------------------------------------------*

Skyscraper in Cells a-b-c-d => - 7 r2c1, r3c8 and the puzzle solves in singles from there (called stte for short). A link to a write-up on Skyscrapers is this.

Leren

- Leren
**Posts:**2894**Joined:**03 June 2012

Thank you both,

JC, my analysis should be more complete. Posted this 1 too early.

Leren, I can't believe I keep missing these skyscrapers. The logic behind it is very clear so it is quite frustrating that almost every puzzle I post is solved with a skyscraper, haha.

I was wondering if one of you has a tip for me regarding search strategies? One downside of the app I'm using is that it does not highlight candidates. That could be a first step, start using for instance Sudoku Joy by Jason ... which does have this function.

But I was wondering, what is your strategy when you start doing a puzzle?

Now, I start looking for naked singles, bivalues and bilocals and then obeying Keith's principle (thanks to JC). When this doesn't result in anything anymore I start filling in all candidates and then look for patterns. Is this a right way to do it? I know i should develop my own style but i always am interested in how more advanced people work.

Furthermore I had some other questions:

I know this can differ completely per puzzle but what are your guidelines when you try to solve a puzzle? What are the most important strategies to look for?

When, of a certain digit, there is only a fish left (for instance swordfish 3), is there anything we can do with these candidates or is it necessary to solve other digitis and the solution of the fish comes by itself?

And as an elongation of the previous question: when I see an "almost" fish of a digit, lets say I find an almost jellyfish 6 (for example I have 3 rows which are organized good but then the none of the last 2 rows fits), does this implicate there is something that can be deduced from the candidates of this digit?

Thank you for your time in advance,

Daniël

JC, my analysis should be more complete. Posted this 1 too early.

Leren, I can't believe I keep missing these skyscrapers. The logic behind it is very clear so it is quite frustrating that almost every puzzle I post is solved with a skyscraper, haha.

I was wondering if one of you has a tip for me regarding search strategies? One downside of the app I'm using is that it does not highlight candidates. That could be a first step, start using for instance Sudoku Joy by Jason ... which does have this function.

But I was wondering, what is your strategy when you start doing a puzzle?

Now, I start looking for naked singles, bivalues and bilocals and then obeying Keith's principle (thanks to JC). When this doesn't result in anything anymore I start filling in all candidates and then look for patterns. Is this a right way to do it? I know i should develop my own style but i always am interested in how more advanced people work.

Furthermore I had some other questions:

I know this can differ completely per puzzle but what are your guidelines when you try to solve a puzzle? What are the most important strategies to look for?

- I know a lot of the basic strategies now (UR, BUG, Remote pair, X-wing, Swordfish, Jellyfish (finned and sashimi for the most part), ER, 2-string kite, X-Wing, XY-wing and the list goes on... Since I'm very busy with my study I didn't have much time recently, but i have to start looking into the chaining strategy category. I think there is a lot of terrain to win in that domain.

When, of a certain digit, there is only a fish left (for instance swordfish 3), is there anything we can do with these candidates or is it necessary to solve other digitis and the solution of the fish comes by itself?

And as an elongation of the previous question: when I see an "almost" fish of a digit, lets say I find an almost jellyfish 6 (for example I have 3 rows which are organized good but then the none of the last 2 rows fits), does this implicate there is something that can be deduced from the candidates of this digit?

Thank you for your time in advance,

Daniël

- Sudoku123456789
**Posts:**34**Joined:**10 September 2017

Hi Daniël,

I think that we get more enquires that are solved with Skyscrapers than anything else, I suppose that's because they are the simplest move that is not discussed on many other sites.

To me looking for a Skyscraper should be no more difficult than looking for an X Wing. Look for two rows or columns that contain exactly two instances of the same digit.

If you find two such rows and in the columns one digit set lines up and the other doesn't you have a Skyscraper. If the digit sets line up in both columns you have an X Wing. Obviously similar comments apply to Skyscrapers in columns.

Regarding solving order, you will never get the same answer from any two solvers, but here are some comments for what they are worth.

On this forum we have the concept of basics. These consist of singles, pointing pairs and triples, naked and hidden pairs, triples and quads.

After that I'd suggest that you look for basic fish and possibly the more obvious UR patterns. After that you will have to look for AIC's. The simplest of these are XY Wings, Skyscrapers, Empty Rectangles and Kites.

Regarding "almost" fish, I think you are talking about finned fish. This extensive topic is well written up on several sites, including here, here and here.

I think that we get more enquires that are solved with Skyscrapers than anything else, I suppose that's because they are the simplest move that is not discussed on many other sites.

To me looking for a Skyscraper should be no more difficult than looking for an X Wing. Look for two rows or columns that contain exactly two instances of the same digit.

If you find two such rows and in the columns one digit set lines up and the other doesn't you have a Skyscraper. If the digit sets line up in both columns you have an X Wing. Obviously similar comments apply to Skyscrapers in columns.

Regarding solving order, you will never get the same answer from any two solvers, but here are some comments for what they are worth.

On this forum we have the concept of basics. These consist of singles, pointing pairs and triples, naked and hidden pairs, triples and quads.

After that I'd suggest that you look for basic fish and possibly the more obvious UR patterns. After that you will have to look for AIC's. The simplest of these are XY Wings, Skyscrapers, Empty Rectangles and Kites.

Regarding "almost" fish, I think you are talking about finned fish. This extensive topic is well written up on several sites, including here, here and here.

- Leren
**Posts:**2894**Joined:**03 June 2012

Thanks Leren,

Regarding one unanswered question (on a quest for confirmation): when of a certain digit only a Fish remains, does this imply no further deductions can be made from that digit?

I actually think this statement is a little bit too black-on-white, but I've noticed this from personal experience.

Regarding one unanswered question (on a quest for confirmation): when of a certain digit only a Fish remains, does this imply no further deductions can be made from that digit?

I actually think this statement is a little bit too black-on-white, but I've noticed this from personal experience.

- Sudoku123456789
**Posts:**34**Joined:**10 September 2017

Not quite sure I understand the question but I'll give an answer anyway.

When encountering a finned fish you can make a list of Potential Eliminations (PE's).

These are the fish eliminations that could be made if the fin was false. With a finned fish only the PE's that can see the fish (either directly or via some inference chain) can be eliminated.

If the fin is subsequently eliminated (by some independent move), then obviously the remaining PE's can be eliminated because you have an un-finned fish.

Leren

When encountering a finned fish you can make a list of Potential Eliminations (PE's).

These are the fish eliminations that could be made if the fin was false. With a finned fish only the PE's that can see the fish (either directly or via some inference chain) can be eliminated.

If the fin is subsequently eliminated (by some independent move), then obviously the remaining PE's can be eliminated because you have an un-finned fish.

Leren

- Leren
**Posts:**2894**Joined:**03 June 2012

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