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Look more closely at your 9s.

In box 1 they can only go in r1, so the 9 in r1 of box 2 can be eliminated.

In box 5 they can only go in c4 so can be eliminated from c4 of b2 as well.

That only leave one place for a 9 in b2.

This is called locked (pointing) candidates.

Hodoku is where you can find everything you need for solving sudoku's. From the most basic tactics to the most advanced ones, all explained with examples.

They have a sudoku generator and solver as well. You can select the tactics you are proficient with or would like to train and it will generate a sudoku solvable with those tactics. And when you're stuck you can get vague hints, concrete hints or step by step solutions on how to remove possible candidates and solve the sudoku.

It's an amazing tool much better than any others out there, the only downside might be that it's an offline tool. So you'll have to download it.

http://hodoku.sourceforge.net/en/index.php

There's a string of three W-wings can get you going all kinda involving the same cells.

1) R6C3 and R7C7 together see all 2s in Row 5, so they can't both be 2. So any cell that sees both cells can't be 4, eliminating 4 from R7C3.

2) R6C3 and R7C7 together see all 4s in Box 7, so they can't both be 4. So any cell that sees both cells can't be 2, eliminating 2 from R7C3. This makes R7C3 a 5.

3) Now R8C3 and R7C7 together see all 2s in Row 9, so they can't both be 2. So any cell that sees both cells can't be 4, eliminating 4 from R7C2 and R8C7. This leaves a single 4 in Box 7 and Box 9.

There could be easier things, but this was the first thing I spotted and W-wings are one of my favourite techniques.

The hodoku program help has a clearer explanation (plus lots of other techniques, but:
* only one of r1c4 & r1c7 can be the 2 (it's also possible that neither are).
* if r1c4 is the 2, then r1c7 cannot be and r7c7 has to be.
* if r1c7 is the 2, then r1c4 cannot be and r9c4 has to be.
* if neither r1c4 or r1c7 is the 2, then both r7c7 and r9c4 are 2.
* since either r7c7 or r9c4 (or both) must be the 2, then r7c6 cannot be 2.
that should help you finish this one.

*Edit: moved to be a reply.

You'll need to use some notation to see where numbers can go...

...once you do you'll find an XY-Wing (a.k.a. Y-Wing, Bent Triple) which will set r5c4 ("5D" here) and the puzzle will be singles form there.

Alternately, a W-Wing will set r6c5 and again, the puzzle solves from there.

It can't be BUG+1 because I think all the cells must be bivalue cells except one cell has 3 possible candidates. At least at this point in the puzzle. there may be one later.

Take a look at what kirlappeee is saying. It looks like a UR Type 1:

http://hodoku.sourceforge.net/en/tech\_intro.php

Totally. The short explanation is that you have exactly 3 cells in a column that contain exactly 3 numbers between them, and therefore those three numbers can't appear anywhere else in the column. Try putting an 8 in R6C9 and see what happens. Here's a longer and potentially more confusing explanation of the Naked Sets technique: http://hodoku.sourceforge.net/en/tech\_naked.php

It looks like you're missing some basic techniques.

http://hodoku.sourceforge.net/en/techniques.php

I'd say read up through x wing. From there you should be able to solve most generated puzzles. (Most phone apps never get more complex than this)

After that you would advance your repetuiore with tricks

You won't find techniques other than bifurcating, backtracking etc for the really hardest Sudokus. Even XY-chains and loops can be very difficult and complex.

Here is a pretty comprehensive list as implemented in Hodoku: http://hodoku.sourceforge.net/en/techniques.php

Sudokuwiki's list: https://www.sudokuwiki.org/Strategy_Families

Im also pretty new to the Skyscraper method so I would recommend taking a look at this site, it does a pretty good job of explaining how it works and gives a couple examples: http://hodoku.sourceforge.net/en/tech_sdp.php#sk

Why? Because BUG+1. Every remaining cell contains only two potential candidates, except r7c2, which contains three candidates. This implies it must be a 6.

For more in see http://hodoku.sourceforge.net/en/tech_ur.php#bug1

There is a W-Wing.

The two sets of >!45 on r7c5 and r8c9!< are connected by two >!4 in r9!< allowing the removal of >!5 from r8c6!< leaving a >!47 pair in c6!<.

There's a W-wing in R6C5+R9C4 linked by Row 3. R6C5 and R9C4 can't both be 8 as then there is no room for 8 in Row 3, so any cell that sees both cells can't be 9, eliminating 9 from R9C5.

There's a 2-string kite on 1 using Row 6 and Column 6. R6C5 and R4C6 can't both be 1, so at least one of R3C6 or R6C7 must be 1. This eliminates 1 from any cell that sees both these cells, in this case R3C7. From here on you get a whole lot of singles.

Check the explanation in the hodoku docs about hidden rectangles. If only 1 cell (R8C8) of a UR doesn't have extra candidates, then look at the cell opposite it in the UR (R5C9). If only one of that cell's UR candidates (7 & 8) has matching candidates (in this case the 7) in related houses (C9, R5, Box 6) outside of the other cells of the UR, then remove it (leaves 1 & 8 as in the example).

I haven't seen Andrew Stuart's solver, but I use the above technique regularly.

*Edit: forgot the why. If R5C9 were 7, then r8/c8 would be 7, and the other 2 cells of the rectangle would be 8. that would allow 2 solutions, making this an invalid sudoku. *Edit 2 - readability.

Once that all looks good, take a look at this. The idea of a swordfish is the exact same idea as an x-wing but one higher level of abstraction. An x-wing can kinda be summarized like this: there is a candidate in a set of rows or columns where considering the candidate as already existing in those rows or columns will exclude it as a possibility from cells outside those rows or columns no matter where you consider it. Essentially, you have a set of possibilities and every one of them results in a certain candidate being excluded from some cells. The definition is exactly the same for a swordfish, however the number of cells is 2 or 3 instead of 2. In a swordfish, rather than finding 2x2 rectangles, you need three rows or columns that have that candidate three or less times to form a grid (it won't always have to be a grid but it's easier to understand it this way). The 4s that are in the green circles form a swordfish. When you consider the possibilities of a swordfish's cells you end up with either a direct elimination (like the first scenario of the x-wing) or you'll end up with an x-wing (or one extra step to direct elimination). In the image, here's the way it plays out, I'll use row 2's possibilities for 4 and I'll show you how r6c3 can't be 4 (you can apply the same process to any of the cells with a 4 in a red circle). In row 2, the 4 can be in column 2, column 3, or column 5. If it's column 2, then the 4s that are still possible in row 4 and row 7 form an x-wing that excludes 4 from r6c3. If it's in column 3 then it directly excludes it from r6c3. If it's column 5 then r7c4 can't be 4 so the only possible 4 in row 7 is in column 3 so 4 can't be in r6c3. No matter where the 4 will be in row 2, it is impossible for there to be a 4 in r6c3 or any other cell that crosses the swordfish.

Google Play (Android) iTunes (iOS)

During the last COVID lockdown I didn’t know what to do with my time, so I created this little Stradoku app called Guriddo (グリッド, japanese for grid).

The rules of Stradoku are similar to Sudoku: You have a 9x9 grid that has to be filled with numbers from 1-9, however there are no subgrids. The grid is divided by tricky black fields. But see for yourself.

If you have any further questions, suggestions, praise or criticism, do not hesitate. Also which features would you like to see in the future?

I hate to plug my own book, but since you're asking you should try Beyond Puzzled! All puzzles are originally designed and have a very broad range of difficulty. The puzzles are designed such that they become more difficult as you progress through the book. There are tons of different unique styles of sudoku to add new levels of difficulty to classic sudoku. Although it isn't spiral bound, I have not had any complaints about comfortability. I have gotten great reviews from sudoku fans and avid puzzle solvers. The book is available on Amazon using the link below.

Beyond Puzzled

If you do decide to purchase, I'd love your feedback! Happy solving!

Well, I'm certainly biased, as I was in the same place like you and developed my own solution.

Recently, I posted a description of the disadvantages I saw in most other apps and a list of features.

EDIT: corrected link

Warning - self promotion: I found the sudoku programs that give you "hints" by solving - more or less - random cells and don't give the reason for it quite annoying. So I implemented my own improved version:

If you have an Android device, you could try it: Sudoku Dojo - it gives you textual and graphical explanation for each hint that you request.

It also

• allows you to use conjugate pairs/coloring
• creating (or entering) sudokus, so that you could enter this sudoku and use the hint feature to get an explanation

There is currently no naked or hidden single in this game - meaning there is no cell that has only one remaining candidate (naked single) and no house (i.e. row, cell or box) that has a candidate in only one cell.

So /u/Pistolcrab's advice to fill in pencil marks is the way to go. Once you have done that, you could start to eliminate candidates until you find singles.

Examples:

• The first row contains the candidate 7 at the positions r1c4 and r1c6. Both positions are in box 2. As each row has to contain each candidate, we now know that the 7 in the second box has to be in the first row. Even though we do not know which of the 2 positions is correct, the candidate 7 can be eliminated from two other cells in box 2 (r3c4 and r3c6).
• More or less the same applies to candidate 6, column 2 and box 4: The only remaining positions of 6 in column 2 are r5c2 and r6c2, both of which are in box 4. Thus no other cell in box 4 may contain 6.

If you happen to own an Android device, I'd be very happy, if you gave my app (Sudoku Dojo) a try. As soon as you have entered pencil marks (or let the app do that for you), it'll give you logical hints, i.e. a full description and graphical display of the logic needed to make the next step towards the solution. I've posted recently about its features.

I see a few things:

• Skyscraper on candidate 5 in R3C58 and R5C68 eliminates 5 from R12C6 and R4C5.
• Skyscraper on candidate 6 in R18C8 and R28C4 eliminates candidate 6 from R1C6 and R2C79.
• Y-Wing on candidate 4 in R3C4, R5C4, R6C6 eliminates 4 from R4C4.

X-Wings are part of an interesting class of puzzle-solving techniques known as "fish":

http://hodoku.sourceforge.net/en/tech_fishb.php

The one you really want to learn at some point is Skyscrapers. Skyscrapers are very similar to X-Wings with a slightly different configuration. But once you start seeing Skyscrapers you see them everywhere all the time.

You're kind of doing a w-wing, but somewhat in reverse. R3C4 and R9C6 together see all 4s in Box 5, so they can't both be 4. So any cell that sees both cells can't be a 7, eliminating 7 from R13C6 and R9C4, leaving R3C4 and R9C6 as the only 7s in Box 2 and Box 8.

Actually, I have another one. There's something called the x-chain (see here - http://hodoku.sourceforge.net/en/tech_chains.php )

Look at this image I've posted. What you do is you start with a number and you find connections to another number but you have to do it like this:

-starts with a strong link (only two in the row, column, or house)

-then you can do weak or strong link

-then a strong link

-another weak or strong link

-a final strong link

Then if the number that you landed on (8 for example) sees another 8, and that 8 also sees the original 8 you started from, you can eliminate it.

So when I'm doing this I start with a number and just go "strong, weak, strong, weak, strong--" and then see if it sees the same number as where I started. And by "sees" i mean if it is the same row, column, or house. You can see that in the image I posted the 8 that we started with is in the same column and the 8 we ended with is in the same row.

https://imgur.com/a/Htv6rah

Thanks!

I was referring to the "Chain" section. The app doesn't (yet) know the special terms for some of the chains. The skyscrapers will be the chains of length 3 where the green lines are parallel :). So at that length, chains 2 and 6.

You can read about what an AIC is, how type 1 and 2 work, and also continuous nice loops at the Hodoku website here. There are probably other sources, too.

I keep slowly adding more explanations to the app. Maybe one day people won't need to ask anymore :). Until then, thank you for asking!

I agree with you assessments. :)

This is a grouped continuous nice loop. All of your weak links do convert to strong ones, but that doesn't end up giving you any eliminations in this case.

I can confirm what xemnosyst is saying. UR's are dictated by the constellations of starting givens as well as the values we pick for cells. So as long was one of the four you have mentioned is a given, then the UR situation will most likely not exist.

Here is really good wiki page on the 10 most common uniqueness techniques:

http://hodoku.sourceforge.net/en/tech_ur.php

Notice how the grids do not show any of the four cells having a given. I think having the given prevents the ambiguity from ever happening.

You kind of used a Skyscraper. It's at least the simplest way to represent the elimination. It's actually easier to look at it from Column 7 as working from Column 8 just adds an extra (unnecessary) step to it.

Row 1 and Row 6 both have two options for 6. As R1C7 and R6C7 can't both be 6 at least one of R1C6 and R6C5 must be a 6. Thus any cell that sees both R1C6 and R6C5 can't be a 6, eliminating 6 from R4C6.

I made an app to find strategies you could use when you're stuck. You can see this game in it here, and check out the help menu.

There are a couple unique rectangles to find on this board! Not the simple ones (type 1), but harder ones to spot. The other kind I look for are hidden rectangles, which I learned about from here.

There are also, of course, chains. There are (almost) always chains. So if you get good at those, I bet you'll get past those diabolical puzzles.

When I started learning a lot of these techniques, I used Hodoku. Shortly after I downloaded Andoku from the Google Play Store and started solving puzzles in that. It has a lot of really helpful tutorials for techniques. Those techniques (hints) available while solving scale to the difficulty, e.g. you won't learn about Jellyfish while solving Easy puzzles. Andoku also has the ability to compute candidates for you at the harder levels and solve singles as well. It's glorious.

I'm sure you can find docs. I use the hodoku app -> help is here. Below is a chain that works on this sudoku. It's a bit longer than average, so it may not be the right choice for learning about XY chains

if r4c5 = 9, then r1c5 = 7 if r1c5 = 7, then r1c6 = 8 r3c6 = 5 r3c4 = 2 r7c4 = 7 r7c6 = 4 r7c3 = 2 r1c3 = 4 r1c1 = 2
else

r4c5 = 2
r4c1 can see both ends of the chain so it can't be 2

The more I look at it maybe there's some easier technique to learn.

Naked Subsets and Hidden Subsets would be the first place to start.

In my opinion in general this is a technique you're better off familiarising yourself with in puzzles below expert level as they're usually not enough to get through many expert level puzzles. This puzzle does have a bunch of them at this stage of the puzzle, but I haven't looked further through this puzzle to see how far you get with them.

>I can’t figure out why the c9 eight eliminates the r4 eight and not vice versa. I hope that makes sense.

They do eliminate each other. The pattern you've identified can be seen as two separate x-wings depending on which one (green or red) is assigned as the fin. If red is the fin, green can be eliminated. Same is true when the colors are reversed.

Interestingly the 8s in rows 2&6 form two sashimi x-wings, with either r2c7 or green as the fin. This is also commonly known as a skyscraper.

There’s a w-wing of >!4/5 in r3c1 & r5c8!< connected via the strong link of >!4s in row 6!< that removes >!5 from r3c8!<.

Basically there can only be >!two 4s in row 6 (r6c1 or r6c8), whichever it is you will always end up with a 5 in either r3c1 or r5c8. So you can remove 5 from any squares they both see (r3c8). !<

You can read more about the technique here.

Oh I think I recognize it now. It's a disconcinuous nice loop, but where you have to use a group link for those two 7s in the top right. :)

You threw me for a loop (no pun intended) because I always start with considering that something is NOT a particular number, and you started with considering that it IS a 7. But I can see it as that discontinuous loop starting with "r8c8 is not 9".

Assuming all of your candidates are correct, I see a unique rectangle (type 1), which breaks the puzzle.

Candidates: >!4 and 9 in r2c1 and r2c2 and r4c1 and r4c2!<

Eliminations: >!4 and 9 from r4c1!<

I would NOT say it's the "wrong" way. I'm fine with it. I was just trying to understand how you figured it out. I think most of the chaining techniques are a bit of trial and error working down the puzzle with bivalues as starting points. I think the technique you used is called a "Forcing Chain":

http://hodoku.sourceforge.net/en/tech_last.php

The four corners of your rectangle are each located in different 3x3 boxes. So imagine an almost solved grid where only these four cells are remaining. It would definitely not be ambiguous, since in each of the boxes only a single number would be missing.

In order for such a combination of hinted cells to form a Unique Rectangle (the kind of deadly pattern you're looking for), they must also be confined to exactly two 3x3 boxes. Then even solving all other cells does not help you to determine the ordering of the two pairs and the Sudoku has multiple solutions.

In your specific case there is another, larger deadly pattern to avoid that you can use to solve the Sudoku: The Binary Universal Grave.

If you want to solve the puzzle without relying on uniqueness, look for a Y-Wing >!in boxes 1 and 4!<.

Hodoku is a great tool for practising with. It has a learning mode that lets you select which techniques you want to practise and it gives you partially solved puzzles where those techniques are immediately applicable. I also found that I rarely need to look outside of their technique descriptions to understand how they work.

Yeah that app suggestion is called bowmans bingo aka guessing and is a bad idea!

This website is a great place to learn beginning to advanced concepts.

http://hodoku.sourceforge.net/en/techniques.php

There are a lot of YouTube videos out there that explain them as well.

There is another way of looking at it. (since it causes contradiction)

If R3C3 is 8 then R3C2 is 9. Then R4C2 is 8. Also R6C2 will be 3. So the 9 in box4 can go only in R4C3. This forms 89 deadly combo.

As per hidden rectangle definition in http://hodoku.sourceforge.net/en/tech_ur.php, 8 can definitely be removed from R3C3.

If you put a 7 in either of R78C3, then the other cell will be an 8, and then you'll also have 7 & 8 in R78C6, which will form a 7-8-7-8 non-unique rectangle.

http://hodoku.sourceforge.net/en/tech_ur.php#u4 might help you understand it.

In rows 2 and 3 of column 3 (which are both in box 1), we don't know which cell will hold the 3 and which will hold the 7 - but we know for a fact that those two numbers will be in those two cells, so any other cell in that box (and in column 3, if there were any) that have a 3 or a 7 can have those 2 numbers erased as candidates.

See an explanation of a naked pair here.

A Remote Pair on four of the 2,8 pairs (starting at r6c3 --> r8c3 --> r8c7 --> r5c7) allow you to remove the 2 and 8 candidates from r5c1, leaving only one 8 in the row and the rest of the puzzle as singles.

Regardless of which number you pick in r6c3, r5c7 will always be the other, so any cell that sees both of those cells can have neither candidate.

There's a W-wing using the >!digit 8!< in >!Row 6!< and >!R3C9 and R5C5!<.

Full explanation: >!R3C9 and R5C5 together see all 8s in Row 6, so they can't both be 8. Any cell that sees both R3C9 and R5C5 therefore can't be a 5, eliminating 5 from R3C5.!<

The basic idea is to find two columns or rows where there are only two instances of one digit in each line. One digit in each line must be able to see each other. You can use the ends of the skyscraper to eliminate candidates both ends can see.

You can read about it here, let me know if you have any questions.

http://hodoku.sourceforge.net/en/tech_sdp.php

In column 3, there's a 14 pair (in rows 1 and 4...r14c3), so 1 and 4 can be removed from the rest of the column.

That leaves a 23 pair in r56c3. In box 5 (r456c456), the 2s are locked in to r56c5. Because of uniqueness, we cannot have a 3 in either of these places, which only leaves r6c6 for a 3 in that box.

It should fall apart from there.

&#x200B;

Uniqueness: http://hodoku.sourceforge.net/en/tech_ur.php

BUG, as I understand it, is the end result when everything is solved except for a remaining “unique rectangle.” It means the puzzle has two valid solutions, which means the puzzle is poorly designed or isn’t a proper sudoku. http://hodoku.sourceforge.net/en/tech_ur.php

There's a Simple Colors link containing 3 single links. http://hodoku.sourceforge.net/en/tech_col.php

I typed it into Andoku to show it to you. Do not click if you don't want to be spoiled:

DO NOT CLICK IF YOU DON'T WANT TO BE SPOILED ANY FURTHER

You might want to try Hodoku. Runs on a computer (I've run it on both Windows and MacOS). Has lots of bells and whistles, but most important it supports multiple difficulty levels, pencil marks, hints (vague, concrete, and walkthrough), reasonable docs for many techniques, and more.

As far as your current puzzle goes, since you have no pencil marks, I'm not sure how to explain a kite. I'll just say that r3c3 is an 8.

So, read about sashimi X-wings here, and here. A normal X-wing basically makes a 2x2 grid where all other candidates can be removed. In a finned or sashimi version, one of the corners of the 2x2 is missing.

Now, because the corner is missing, you can't use it normally. You can still make eliminations like it was a standard x-wing, but only in the block where the omission occurred. That's why it's called a sashimi; it's an X-wing with a piece hacked off. You still need one or more 5's in that row, but they can be in any arrangement.

So let's take the first sashimi. You have 5's in r5c5, r5c9, r8c4, r8c6, and r8c9. This is not an X-wing, because you're missing a corner in r8c5, right? Well, that's fine for a sashimi. Imagine a 5 was there, and you can make eliminations in block 8. The only removal in that case is the 5 is r7c5.

The same process works for the 8's in rows 3 and 8. Heck, the cells in row 8 are even the same, to make it easy. Put both of these together, and you've removed 5 and 8 from r7c5.

Also, consider that a sashimi X-wing is one of the easier advanced techniques. X-wings and their derivatives (kites, towers, mutants, etc.) are a cornerstone to advanced solving. The others are probably things like XY-wings, XYZ-wings, and XY-chains. To a certain extent, you'll need W-wings, BUG+1, X-cycles, and so on, but you get the idea.

I won't even mention ALS-XZs or AICs, because they're much more complicated and require a far deeper understanding to how puzzles operate. The techniques I mentioned should serve until you get to the hardest of puzzles, and then all bets are off.

I made a this picture that will hopefully explain a little better. We don't worry about the 8 in r5c8 because we can exclude the 4 and 7 from r5c7 leaving us with 8 as the only option in that cell. In case I still didn't do a good job explaining, the technique is called a triple and you can read more about it here

Techniques I recommend (easy enough to learn and generally the least annoying to look for):

Naked/locked sets

Swordfish

Skyscraper/siamese/sashimi fish

Remote pairs

Empty rectangles

2 string kite/turbot fish/small x-chains

Unique rectangles

BUG

W-wings

Xyz-wings

Xy-wings (sometimes annoying but important to know)

http://hodoku.sourceforge.net/en/techniques.php

I like this one. The UI is actually clean, contains ads but are those which are shown when you finish a puzzle - but can't tell if there are not solvable puzzles or it's just me being a dumbass - though I could solve a few ones in the 'extreme' mode

Open Sudoku

No ads, rather basic (not pretty). But I prefer it because of the good import support. I bulk generate my own puzzles using Hodoku and import them into Open Sudoku.

https://play.google.com/store/apps/details?id=com.onegravity.sudoku.sudoku10kfree&amp;hl=en_CA&amp;gl=US

This is the free version of the app I've been using for quite some years now. I can't speak for how intrusive the ads are because the paid version doesn't have them.

It supports full cell highlighting like you're asking, puzzle analysis, hinting, generation, colouring and reasonably defined difficulty levels. My only complaint is that entering in an existing puzzle from another source (like /r/sudoku) is so convoluted that I'd rather go to my computer for it.

Sudokumaniacs.com has 1 classic & 1 variant a day with leaderboards. I actually play on the website everyday (I'm faster with a mouse & keyboard rather than on a phone), but they do have an app: Sudokumania

I think I've mentioned this website just a day or two ago, so I should probably state that I have no affiliation with the developer, I just really like the work. It doesn't hurt that a few of the world's fastest solvers (including a former world champion) play there every day.

I just started playing a couple months ago and this is the first app I downloaded.

https://play.google.com/store/apps/details?id=easy.sudoku.puzzle.solver.free

I like it fine but can't really compare it to anything.

I use this one: SudokuChess
It offers classic sudoku and anti-knight and anti-king ones. It's the only one so far aside from Cracking the Cryptic, that has that same notation and allows for easy change of corner to middle marks. :)

My current favorite is not a traditional Sudoku, but an interesting spin on such. It is called SudoCubed, and is pretty much Sudoku expanded to cover the faces of a cube.

Each face of the cube is a Sudoku puzzle, but the edge cells overlap. So you have to rotate the cube to and peek around the other side to see if a digit placement is valid. I really like it, pretty fun twist.

Google Play: https://play.google.com/store/apps/details?id=com.highwaynorth.sudocubed

App store: https://apps.apple.com/ca/app/sudocubed/id1450212225

Look into better erasers. Like from an art supply store. They have erasers that load into a holder that works like a lead pen. You can even get a fine line eraser.

I wouldn't worry too much about trying to find an app with the colouring feature; it isn't that common or popular/necessary of a technique (I only see a couple people in this community use it, and just one that advocates for it HARD). I would focus on the basics (hidden or naked singles/pairs/triples, pointing pairs, etc) and from there you can build to other techniques (x/y/xyz wings, fishes, etc).

I don't often play on my phone, but I have the andoku 3 app and like it. Again, it doesn't do colouring (that I know of), but when you select a number, it will highlight all of them that are on the board (both the determined numbers, as well as candidates). The highlighting feature, I think, is more beneficial in that it can help you see certain patterns.

https://play.google.com/store/apps/details?id=com.andoku.three.gp&amp;hl=en_CA

This one does. You can even take a photo of the puzzle and the app will detect the numbers for you.

https://play.google.com/store/apps/details?id=com.enjoysudoku.enjoysudoku

I've been using this app for a while, and many friends have asked me about it. It's beautiful and simple, give it a go. https://play.google.com/store/apps/details?id=ee.dustland.android.dustlandsudoku

Check out Andoku 3. It has tutorials for various levels of techniques and when you request hints it explains why you get the hint you get.

https://play.google.com/store/apps/details?id=com.andoku.three.gp

Surely you didn't try Enjoy Sudoku then? :)

Also has a free version.

Great board visibilty, step-by-step tutorials, massive difficulty range, examples of techniques, hints explaining techinques used , time spent solving compared with previous solvers, statistics and more!

My favorite is probably Enjoy Sudoku. I've probably tried dozens of them, and it always comes out on top. The interactive tutorials were what helped me learn several of the advanced techniques needed for difficult puzzles, too.

Edit: Linked.

We have just released a new free Killer Sudoku app to the app stores. We hope you love it.

The puzzles are handcrafted and divided to 5 levels of difficulty from beginner to expert. The app has rich UI, that include:

* Center, side and double notation for pencil marks.

* KIller calculator

* User defined cages.

* Smart hints.

* Highlights

* Auto fill pencil marks

* Weekly challenges

* Many more features

We need your support to make the app a success. Thank you in advance.

Android download: https://play.google.com/store/apps/details?id=com.uvmlab.killer

IOS Download: https://apps.apple.com/app/killer-sudoku-by-logic-wiz/id1641151077

Check out SudoCubed, it is Sudoku using each face of a cube. Single solution for the entire cube. Edge cells are shared with adjacent cube faces.

Apple: https://apps.apple.com/ca/app/sudocubed/id1450212225

Android: https://play.google.com/store/apps/details?id=com.highwaynorth.sudocubed

It's actually not Sudoku.com, just another (very popular) app that seems to imitate the look very closely. Extremely confusing, and the hint is bad either way.

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How about try my app sir? App name is theme sudoku,

All puzzle in my app (except chaos difficulty mode) Are proven solved with logical skill and my app support logical hint for ad-free

I recently launched this app and it will be appreciated if you tell me how you fill sir :)

https://play.google.com/store/apps/details?id=com.URgames.ThemeSudoku

Ah i think i found it - Cracking the Cryptic app - it's an app tho, not a website. So i might still check out Sudoku Solutions. Thanks so much for the help!

https://play.google.com/store/apps/details?id=com.onegravity.sudoku.sudoku10kfree&amp;hl=en_US&amp;gl=US

This app is really customizable

Here is the Android app on Google Play:

https://play.google.com/store/apps/details?id=com.mnemonemo.sudoku

Just drag the pencil over the grid

Good job! With harder Sudokus, you will need more advanced techniques to make progress. Refering to them by name is much more efficient than explaining their logic each time, since there are many of them and the descriptions can get lengthy. I really like the explanations on HoDoKu and SudokuWiki.

https://imgur.com/a/RpR3Yyo

You cannot place BOTH 2 and 8 into the lower red box in the picture above as you would create a rectangle pattern of 2’s and 8’s and the puzzle would have more than one solution. This means that 7 must be placed inside the red box.

More about the technique here: http://hodoku.sourceforge.net/en/tech_ur.php

The sashimi swordfish is on 4s in columns 2, 3 and 6. It is almost a swordfish, except for two additional "fin" candidates (r7c2 and r9c2, orange highlighting) and some omissions (r2c6, r4c2, r8c2).

The logic of finned swordfish works regardless of omissions: Either one of the fins contains the 4, or the rest of the candidates in columns 2/3/6 form a swordfish in rows 2/4/8. In either case r8c1 cannot be a 4.

The second elimination in r7c4 follows directly from the first, because the 4 in row 8 is pushed into box 8. Alternatively you could also see it as part of the sashimi-fish logic: Either 4 is in the fins, which then form an X-Wing with column 8 and eliminate r7c4. Otherwise the swordfish has only one candidate left in row 8 because of the omissions, so r8c6 must be 4, which also eliminates r7c4.

You can make pencilmarks to see what numbers could possibly go where. And that is usually enough for beginners. I'm not going to give a big explanation in here so DM me if you need help. BUT, I recommend taking a look at this page:
Hodoku Sudoku Tactics
Pretty much every sudoku tactic there is, is explained on this page. I'd recommend to at least read the first 4 categories. Singles - naked subsets if you want to get a good grip on solving the average sudoku.
There are wayy more in-depth tactics, but start out simple.

In hard Sudoku puzzles, progress is measured better in candidate eliminations than in final digits entered. Sometimes to have to apply multiple techniques each eliminating one or two seemingly random candidates until the next digit reveals itself. But in this case the triple in column 8 also reveals a pointing pair of 7s in box 9, which leaves only one position for 7 in box 8.

After you've eliminated 3, 7 and 8 from r9c8 because of the naked 378 triple in column 8, the 3s in row 9 can only be in box 7. This means that r8c1 cannot be 3 (and therefore must be 2), otherwise there is no place left for 3s in the last row (Locked Candidates Type 2.

The area that has the hidden triple will also have a matching naked pair/triple/quad/etc.

If you look at the first hidden triple example at http://hodoku.sourceforge.net/en/tech_hidden.php notice how there is also a naked quad, and in the second example, there is a naked set of five.

So I find it easier to try and spot the naked sets, rather than the hidden ones.

Imagine if a 4 was in r4c7. The 6s would be forced into the adjacent corners of the r14c78 rectangle and a 4 would be forced into the opposite corner. This is a non unique arrangement since the 4s and 6s could be swapped to get another valid solution. So to avoid that the 4 cannot be in r4c7. Same goes for the 4 in r4c8.

UR Type 4

Your complaint ought to be about a foul word unless you are considering chickens, but I'd agree, it was uncalled for.

Your problem with this puzzle is that your generator isn't excluding unique rectangle cases. It's not explicitly in the rules for solving a sudoku, but if you are of the opinion that the puzzle should have a unique solution then the generator needs to exclude patterns such as this.

There are a whole bunch of variants you need to catch (e.g. A/B, B/C, A/C in 3 rows).

You could remove the ambiguity by ensuring one of the 4 locations is a "given".

See

http://hodoku.sourceforge.net/en/tech_ur.php

for starters.

You can find it on Google Play Store hereSudoku app. It's really the best app on Sudoku for me. I don't think it's available on Apple App Store.

Google Play (Android)

App Store (iOS)

The rules of Guriddo are similar to Sudoku: You have a 9x9 grid that has to be filled with numbers from 1-9, however there are no subgrids. The grid is divided by tricky black fields. But see for yourself.

If you are new I recommend to install the Guriddo app and take the tour at the beginning, but of course you can apply the rules directly:

📏 Place a number from 1-9 in every grey square.
📏 Numbers within columns and rows must not repeat.
📏 Every straight line of horizontally or vertically connected grey squares must contain a number sequence (e.g. 2-3-4-5), but order does not matter (e.g. 3-2-5-4).
📏 You can't write numbers in the black cells.
📏 Some of the black cells contain numbers. Those numbers are not part of a number sequence.

If you have any further questions, suggestions, praise or criticism, do not hesitate!

David Peru's channel (/u/dxsudoku )

https://www.youtube.com/channel/UC2N60jDPloV5_imTYKIVH2Q

He is a regular here and also has a book published which is very good for beginners.

https://www.amazon.com/Taming-Sudoku-Dragon-Comprehensive-Beginners/dp/1736452606

Good explanation. Note that you can see that last paragraph visualized in the same app, in this case as Net 3 in the help menu (link here).

I have not heard this called a "type 7" unique rectangle before. I know it as a "hidden rectangle", which is not-so-clearly indicated by the word "hidden" in that screenshot.

Hodoku has a similar grading system. And you can set the values for each puzzle-solving technique. In other words, Hodoku's rating system is fully configurable which is pretty cool.

http://hodoku.sourceforge.net/en/docs\_cre.php#rating

The 2nd puzzle is cool with three XY-Wings.

How is this one for gnarly:

4.2...9.7.......2.97.5..8......71.....78524.....34......8..7.46.6.......7.9...2.3

The candidate 2s and 6s in boxes 1/2 and 7/8 are the keys.

2-String Kites or Finned X-Wings on either/both will take you to the end.

Yes you're right about the other 9 but it doesn't matter in this case because it only matters that the 9s in row row 7 and column 6 are limited to the UR cells. If you put a 4 in r7c6 then the adjacent cells in the UR would have to be 9s and the opposite cell would be a 4, leading to non uniqueness. Hidden rectangle.

> Is it ok to use more if they do not cross?

Definitions of fish say that you must have the same number of base and cover sets. However, don't let that stop you from swapping one of the cover sets for an extra one to make more eliminations if you want! Just consider them 2 different fish with the same base sets 🙂.

> Do I have to have two cells in a box in the same row or col in order to make a cover set with another base set? And two parallel rows or cols with more than two cover sets-?

The basic rules are:

1. You have the same number of base & cover sets
2. All the base candidates are contained in cover sets.
3. No base set "overlaps" with another (i.e. cells they have in common are not allowed to have the candidate)
4. No cover set "overlaps" with another

As you look for variants, like finned fish, there are FEWER rules, but eliminations become harder. Fins drop rule (2). You can also drop rules (3) and (4) to look for fish with "endo fins" and "cannibalism". However, as you delve deeper into variants the less useful they become, in my opinion, because they are harder to find and understand. It sounds like you're getting comfortable with finned fish, so maybe try to keep the other 3 rules for now?

My source of info about fish (which I consulted when writing these replies 🙂) is hodoku's docs. If you're willing to read slowly and think hard to understand the language used, it's a great resource.

It's a hidden rectangle because the 1's are limited to those 4 cells in an x-cross formation. If either of the 13 cells was a 3 then it would force a 1 into the 2 adjacent cells in the rectangle and a 3 into the opposite cell. So both of the 13 cells have to be 1s.

New apps come out every day. So writing down a couple of recommendation wont do you any good. I suggest you search on your platform of choice.

1. If you have an Android phone go to Google Play. For Huawei go to the App Gallery or look at the recommendations below.
2. For iOS and iPadOS go to their App Store.
3. For web apps I suggest you look at Alternativeto.net or Cloudinfo.guru They have the most up-to-date comments and information.

Probably doesn't help a lot but I think ijudging for yourself is worth the distraction.

We recently released new update for Meow Tower!
This update includes Hard Mode (15x15), Nonogram Theme, and many bug fixes.
Download now ⬇️
Android: https://play.google.com/store/apps/details?id=com.grapetree.meowtower
iOS: https://apps.apple.com/app/id1579122980

You need to learn the seven most commonly used puzzle-solving techniques. These seven are so common I call them the 7-Base techniques and they are: Naked Single, Hidden Single, Naked Pair, Locked Candidates, Hidden Pair, Naked Triple, and X-Wing. I would suggest you always try the 7-Base techniques first before trying any advanced puzzle-solving techniques. The following tutorial goes into each of the seven in good detail:

https://www.youtube.com/watch?v=6WnFkrUt\_10

Also, in my book available on Amazon I go into the 7-Base techniques in good detail and provide you a set of practice exercises for each one:

https://www.amazon.com/dp/1736452606?ref\_=pe\_3052080\_397514860

In the book there are some nice diagrams of the 7-Base techniques. Sorry for the shameless plug of the book but an insane amount of work went into it.

I hope this helps you with your Sudoku journey. You can send me Reddit messages if you have any questions.

Here's my beginners book available on Amazon if you are interested:

https://www.amazon.com/dp/1736452606?ref_=pe_3052080_397514860

Everyone who gets it and reads it says very nice things about so far.

There's a Finned Xwing of 4s in r59c39, the fin is r7c3. The logic says, either the fish (Xwing) is true, or the fin is true.

If the fin cell didn't contain a 4, there'd be a true Xwing, and candidate 4 in r9c2 could be eliminated. Alternatively, if the fin is a 4, r9c2 can still not be a 4, and can therefore be eliminated.

http://hodoku.sourceforge.net/en/tech_fishfs.php#fbf2

I think you have a Sue de Coq in box 5, candidates 2,3,4,5. Should eliminate 2,4 from r6c5.

http://hodoku.sourceforge.net/en/tech_misc.php#sdc

I am just learning about these, so I could be wrong. Here's a great video tutorial on them:

https://youtu.be/9V3qHFapC1c