Advanced Sudoku: Skyscraper = Sashimi Finned X-Wing!

We take a look at two puzzles requested recently and discuss how to resolve the endgame. Techniques discussed include Skyscrapers, X-Wings (both sashimi, finned and sashimi finned) and Uniqueness using 6 boxes (instead of the usual 4).

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[paulrob]

I understand all the logic, the hardest bit is spotting when and where they exist

[cadmar]

This is by far your best tutorial! Well done!!!

[HingleMcCringle]

Been working on a puzzle for hours, finally found a tutorial that really helped!

[benjaminandrews]

Fantastic video, not complicated and straight forward. Love the channel!

[joeyoest]

In the second puzzle, there's also an 'upside down' skyscraper on 4's in columns 3 and 9. That would allow you to eliminate the 4 in R8C7 and place a 4 in R8C3.

[markdisanzo]

I found a skyscraper on the 4s, but it was different than the one described in the vid. The one is I used comprises cells R8C7, R8C3, R5C3, R5C9. Using those as the skyscraper easily eliminates the 4 from R6C7 and R7C9. You can also use simple coloring starting on R8C7 to achieve the same result.

[Pacer]

From the starting position of this video, there is an empty rectangle with 3's, and if you use it, you can eliminate the 3 from r3c7, leaving only a possible 4 in that spot. And that move unwinds the entire puzzle!

[fulvioscarlata]

absolutely great, it's fantastic

[bogbel]

Complicated as hell for me, but I can follow the logic...but spotting them is really hard.

[johnpedersen]

Simple chain solution first puzzle. Testing the odd duck 3 at r8c6 (the only non 1/3 pair on columns 4 and 6) removes all possible 3s in box 9.

Therefore r7c4 = 3 ftw

[ThatGuy-djqr]

Thanks for this great lesson Simon. You have the ability to explain things very clearly!

[MichaelEriksen]

At 12:10 you say: "Either C9R5 must be a 4 or C1R6 must be a 4". I don't think this is right due to the 2 in C1R7. The 2 in C1R7 could lead to 4 in both C9R5 and C1R6.The 2 makes the link between C9R5 and C1R7 ambigouos.
EDIT: But I just realised the effect is the same if both are 4 i.e that C7R6 cannot be a 4. Without the 2 it is "either or", with the 2 it is "either or both" and I see your conclusion is right in both cases. What a joy to learn these techniques 🙂.

[josephle]

Really nice! Thank you for this video.

[justdavelewis]

On the second puzzle: I don’t see how you can remove the 4 from R6C7 with the skyscraper. I can see that the skyscraper roofs can both theoretically contain a 4 due to the 4 in R7C7 and I thought that meant the skyscraper doesn’t work?

If anyone reads this I’d love to understand

[lawrencekallal]

In the first puzzle, there is a simple force test solution with the 34 pair in RC37.

If one tries putting a 3 in that position and follows a couple of forced 3's around, you end up with a 3 conflict in C7 -- the 3 in the bottom right block also wants to go in C7. So RC37 has to be a 4.

That 4 is enough to crack open the puzzle.

[Akhilesh]

How do you know for X wings whether to eliminate candidates from the rows or columns?

[prycenewberg]

The funny thing is, I found a completely different second Sashimi Finned X-Wing than your second Sashimi Finned X-Wing. I looked in columns 4 and 9, which eliminated the three from column 6 row 8...

[playerone]

Still don't get it. I can see it's either a 4 or 7 but still don't know which?

[filmcamera]

surely in the skyscaper at 12:10 row 6 column is a 4 and row 5 column 9 is also a 4, it is not an either or as you state.

[filam]

I think the app I use requires skyscrapers to solve some of the puzzles.

This might be the lesson I needed. Now it's time to practice 😜