Sudoku Tutorial #40 A must know technique. NOT a puzzle. (2023)


PLEASE NOTE. This is NOT a real puzzle. Don't try to solve it as a puzzle. It is a demonstration only of a technique

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For frequently asked questions FAQ

When you have 3 or 4 or 5 or 6 or more empty cells in a row column or block this "outside the puzzle" procedure is VERY helpful.

Frequently when solving puzzles there are several paths to take to solve the puzzle. SO MUCH DEPENDS ON WHAT YOU SPOT AND WHEN. In this video there is a 3X3 of 2,3 and 5. which means you can remove all other small numbers of 2,3 and 5 in the rest of the row. This will also help you come up with a correct solution. Thanks to Jim W for pointing this out.
ALSO note that 8 can only go in column 3, because block 3 has an 8, column 5 has an 8 and column 1 has an 8. See lesson 13 "There's only one place it can go"

NEW you can now go to my web site and see an INDEX OF VIDEOS under strategy headings. Clcik on the number and it will take you to the video of interest.

Thought for the day : "There are several ways to cook an egg"


Hello everyone here we are again, Robin.

This cool guy with another tutorial.

This is number 40 before I show you what I have to share today.

Let me talk about a couple of things.

First of all I've had people say to me.

Why do you bother to put in little numbers? I can do difficult puzzles without having to put them in well, there's, two things I'd like to say about that.

First of all, if you can put the do you, if you can solve a puzzle without putting any of the small numbers in you're, either very clever, or you have a puzzle, which says, it's difficult, but it's, really not now.

If you remember rightly back in lesson, number four I mentioned that if you're doing really easy to medium puzzles, all you need to have in each cell is two small numbers.

But when you get to the stage where you have a really difficult puzzle, you've tried everything.

You can think of duty and BLC are across me technique, 3x3 all those advanced techniques and you're still stuck.


What do you do? Well I'm going to show you? Because this is tutorial and the next one are based on what I would call advanced outside.

The puzzle let's have a look at this.

Okay in this particular situation, let's say, just for an argument to sink that you've got to the stage where wow, what do I do next? Well, this is the time you could probably go to use the outside the puzzle system or technique as I called.

Now, if you look at this puzzle you'll, notice that there are places where there's five or six empty cells in a row column or block.

And my suggestion would be this always look for say, something that's only got three empty cells in a row column or block work those out first then go to any row column or block that's got four empty cells.

And so on, until you got five empty cells and on this particular tutorial I want to demonstrate what you can do with five empty cells.

What I'm going to do now is show you what you can do with as just as an example to show you in this case, it's going to be a row with five empty cells and it's going to be this one here.

Step one, we look along this row and see work out what numbers are missing.

Well, there's a one there's, not a two.

So we put the two over here outside of the puzzle.

Three there's, not a three along here.

We put that over there, four there's, a four already, five there's, no five along this row.

So we can put the five in six is there already, seven there's, no, seven, either of what is seven in eight is there? An eight along here? No there's, not nine.

Yes, we have one.

So we have five empty cells.

And the numbers that can go in those five empty cells that could be a two or three or a five or seven or an eight now that's.

Step one.

Now comes the part where a lot of people will say, I, don't want to be bothered doing this because I've got a computer that will put all the possible numbers in for me.

Well, if you want to go that route that's, one, that's, okay, but the purest Sudoku solver will say, well, part of the skill of solving Sudoku is knowing what numbers doing each.

What possible numbers go in each of these cells.

And that requires a little bit of brain work it's up to you.

Of course, but I'm going to show you if you don't have a computer, if you have a book or something like that and I use books a lot.

So when I start with this cell and we'll work our way across I say to myself, okay, is there one? Yes, there is a one there's, the one that one this.

So you kind of a 1 there is there a -? Ok? - have a look here? Is there a - well here's a 2? So you can't have a 2 there, because you can't repeat a number in a row column or block.

And so we can't have a - what about it.

A 3 is there a 3 in this block is there a 3 down here? No there isn't is there 3, no I don't, see a 3.

So 3 is a possibility for we've got a 4 5.

We've got to find 6.

We've got a 6 if you've got us 5 and a 6, the four in that row means you can't have a for there 5 and this block meet you can't have a fire there or a 6, ok, let's work the.

But the next seven well is there a 7 in this block? Is there a 7 along here? No is there a 7 down here, no there isn't.

So we can put a possibility of a 7 here, 8, well, there's, no 8 in here.

But there is an H down in here.

So you can't have an 8.

So what we're saying now is the possibility of any kind of an on by the way you in this.

So only 2 possible numbers can go there, how many of those - let's do this one now, yeah, we already have a 1.

Do we have it - no we don't mean, you put it to me? Do we have a 3 there's another 3 in here, there's, no, the 3 along there, there's, no, no three down here.

Yes, there is there's a three right there.

So we can't have a 3/4 is spoken for five is a five in here in this block, there's a six in this block, you can not an accountant what about a seven.

Yes, we could have a seven.

And what about an eight is there? An eight in here, no is there an 8 down in here.

No we could have an 8 there 9.

We've already got a 9.

So now we've got two cells.

Now, what about one here? Well, we have a one in this block.

So we can't have a one there.

We have a two in this block because we can't have it - what about a three is there.

A three down in here, no there's, not a three in there, there's, not a three in the block.

So we can put if possible three, four, we've already got a four five.

Well if I look down and there's, no five in here, and if I look down in here, this column there's, no five.

So the possibility of a five is there as well.

Six, we've already got a six seven.

We've already got a seven in this column, 8 and 9.

We both having this same column.

So we have a possibility of a 3 and a 5 year.

Now, let's go over to this one.

What about? Um, what are we missing we're missing a 1 when we gotta want? We don't have a - I don't, see you two in here? So we put it to me what about a three? We don't have a three down in here.

So there's going to be a three what about a four we've got a four five is there a five down in this column? We know there's, not a fight along here, there's, not a five in here.

So yeah, could be a five six when we've already got a six seven.

We've already got seven eight.

We've already got an eight in this block, and we could have a nine is that right? Well, okay, we could have a nine.

We can't have a nine is an on along here.

So miss that nearly.

So that is a two three and a five now let's go to this.

Last one, we don't have a two.

We don't have a three.

We had a one.

We kind of away in yet over one year, two, three, three, four, four, well, we've got a four.

We don't have a five coming up and down in here.

So it has to be a five as well and six, we've got a six and seven.

We've got a seven in this block.

Eight, we've got an eaten block eight nine.

We've got a nine across here.

So 235 are the ones that going now in that particular those cells.

Now what we've done here is that we've we've worked out what possible numbers will be there.

Now, how do we know which ones? Well, here's, the step that we go through first thing is you look along the row of little numbers? And is there a number that hasn't been repeated? In other words, it's only there once have a look closely and did you notice this it's, an 8 in this row and all those little numbers, there's only one place, an 8 could go, which means you be.

It means that you can get rid of all these little numbers here and put a big 8 in there.

We go let's do that make it nice and neat and clean.

So we're gonna put an 8 in there so let's do that so we're on the way.

Now we come to the next step, and you may say, well, what do you do now? Well I could have an 8 there under there, but did what is that going to get me? Well, it won't get you there.

Look we on an 8 there.

We've got an 8 there.

We've got an 8 there.

So if that's the case if this is the case here, we know we have to have an 8 down in here there there or there we can't ever there because of that right, we can't have it there because of that way.

So this will become.


We will be.

We've got something as a result of finding that lady, doesn't always happen that way, but what's that you're.

Now stuck just as an example, you're stuck.

And you say, what do I do now? Well, here's the next recommendation, go ahead and solve the puzzle.

The best as you can and let's say, you come across a new number that helps you let me show you as an example, let's just say for argument's sake, just so you understand what I mean that a 5 turns up here, one of the ramifications of that here we have a left your ever right in this block.

It has to be in the center.

Now we cannot be there because it's a 5 there, which means that this has to be the 5 so let's erase the 3 and the 5 and put in a big fight Wow.

Can you know what's blue today, put in a big five? So we've got a 5 now what's, the ramification of that 5 that means that any other 5 along this row, you can get rid of so we can get now.

Now we can get rid of this 2, this 5 and this 5 and just for fun.

Sometimes you can do this.

We've got rid of the five.

We've got rid of the eight and so on.

Now, when you've got this two three, two three, you have, of course, a matching pair, and we know from previous tutorials and lessons that if you have a matching pair any other two or three in this row can be eliminated, which means that this three can go.

And you finish up with a big 7, okay, there's your big 7.

Now we know from that video 7 could be there and there, but let's move on.

Because we now have a 2 3 2, it's 3 left.

We've only got 2 little numbers left.

And we know that if you get one you're going to get the other one, so we can get rid of the 7 now so you're left with 2 or 3.

Now, just say for argument's sake that as you work on the puzzle, you come up with a 3 here, just to demonstrate what I mean.

Now you have a top.

We have a middle over here.

We have to have a bottom and that's the only place it can go.

So that becomes a 3 now what's the ramification of that.

Well, that means that this freaking go and you're left with a big 2.

So we put that big 2 in and it like most matching pairs.

Once you've got the two there.

We can get rid of that too, and it becomes your three.

Now, what isn't this incredible? What we've done is that we have now solved that whole row based on outside the puzzle system.

Now next tutorial I'm gonna do another one of these, but to show you how it can change again.


What is the 45 rule in Sudoku? ›

The 45 rule is a basic solving-technique in Killer Sudoku. Each house (row, column, nonet) must add to 45 (the sum of the digits 1 through 9).

What is the secret to winning Sudoku? ›

The most common strategies for solving sudoku involve scanning rows and columns inside each triple-box region, removing numbers or squares, and identifying circumstances where just a single number can fit into a single square is the simplest approach to begin a sudoku problem.

Is there only one solution to a Sudoku puzzle? ›

Sudoku is a number puzzle consisting of a 9 x 9 grid in which some cells contain clues in the form of digits from 1 to 9. The solver's jobs is to fill in the remaining cells so that each row, column and 3×3 box in the grid contains all nine digits. There's another unwritten rule: the puzzle must have only one solution.

Can Sudoku be solved without guessing? ›

The short answer is yes. Every proper Sudoku puzzle can be solved without ever having to make a guess. Another way of thinking about it is that every Sudoku puzzle can be solved logically. Even though it may require highly complicated solving techniques you're not familiar with.

What is Sudoku rule of 1? ›

Sudoku Rule № 1: Use Numbers 1-9

Each row, column and square (9 spaces each) needs to be filled out with the numbers 1-9, without repeating any numbers within the row, column or square.

What is the rule of two in Sudoku? ›

The game of Sudoku consists of a grid of nine rows and nine columns subdivided into nine 3x3 subgrids. It has two basic rules: Each column, each row, and each box (3x3 subgrid) must have the numbers 1 to 9. No column, row or box can have two squares with the same number.

What is the Sudoku one rule? ›

The rules for sudoku are simple. A 9×9 square must be filled in with numbers from 1-9 with no repeated numbers in each line, horizontally or vertically. To challenge you more, there are 3×3 squares marked out in the grid, and each of these squares can't have any repeat numbers either.

Is Sudoku good for brain? ›

International Sudoku Day 2022: Playing Sudoku not only improves concentration and memory but also cuts risk of developing Alzheimer's disease. Playing Sudoku can not only improve concentration and memory for children, but for elderly it can keep Alzheimer's - the most common forms of dementia - away.

What is Sudoku rule of 4? ›

There are three rules for Sudoku 4×4. Please remember them all. Each row contains each digit of 1–4 exactly once. Each column contains each digit of 1–4 exactly once.

What are the algorithms in Sudoku? ›

The interesting fact about Sudoku is that it is a trivial puzzle to solve. The reason it is trivial to solve is that an algorithm exists for Sudoku solutions. The algorithm is a tree-based search algorithm based on backtracking in a tree until a solution is found.

What is the best order to solve Sudoku? ›

The easiest way starting a Sudoku puzzle is to scan rows and columns within each triple-box area, eliminating numbers or squares and finding situations where only a single number can fit into a single square. The scanning technique is fast and usually sufficient to solve easy puzzles all the way to the end.

What is the best Sudoku algorithm? ›

The simplex algorithm is able to solve proper Sudokus, indicating if the Sudoku is not valid (no solution). If there is more than one solution (non-proper Sudokus) the simplex algorithm will generally yield a solution with fractional amounts of more than one digit in some squares.

Is there a pattern to Sudoku? ›

Patterns are essentially a series of completed numbers (either givens or numbers that have been filled in) in a specific order in one of the boxes on the sudoku grid. Patterns help you organize the remaining numbers and eliminate possibilities in a box, and in related boxes, rows and columns.

What is the logic of Sudoku? ›

Sudoku is a logic-based puzzle. It is a type of constraint satisfaction problem, where the solver is given a finite number of objects (the numerals 1-9) and a set of conditions stating how the objects must be placed in relation to one another.

What is a unique rectangle in Sudoku? ›

A "Unique Rectangle" (UR) consists of four cells that occupy exactly two rows, two columns, and two boxes. All four cells have the same two candidates left (in real sudokus not all cells have to hold all of the UR candidates, see below).

Can there be different solutions in Sudoku? ›

A well-formed Sudoku puzzle is one that has a unique solution. A Sudoku puzzle can have more than one solution, but in this case the kind of logical reasoning we described while discussing solving strategies may fall short. There are examples of rank-3 Sudoku puzzles with 17 givens that are well-formed.

What happens if I solve a Sudoku daily? ›

The more you play, the better you become. If you play Sudoku daily, you will start to discover that you have almost a sort of “sense memory” for the puzzle grid – you will start to see patterns emerging and develop the ability to seize opportunities more quickly.

What is the swordfish technique in Sudoku? ›

A Swordfish is a 3 by 3 nine-cell pattern where a candidate is found on three different rows (or three columns) and they line up in the opposite direction. Eventually we will fix three candidates somewhere in those cells which excludes all other candidates in those units.

Are 1 and 9 consecutive in Sudoku? ›

Consecutive Sudoku Rules

The rules of Consecutive Puzzles are as follows: Place the numbers 1-9 once in each row, column and 3x3 bold-lined box in the grid. Green bars between squares indicate that the values in those squares are consecutive.

Can we use 0 in Sudoku? ›

Sudoku grid consists of 9x9 spaces. You can use only numbers from 1 to 9.

Can the same number be next to each other in Sudoku? ›

Introduction. Sudoku puzzles cannot have the same number twice in any row, column or box (container).

What are the two dots in Sudoku? ›

Normal sudoku rules apply. White dots between two cells indicate consecutive digits and black dots between two cells indicate a ratio of 1:2. For some rows and columns, the TOTAL number of white and/or black dots appearing in that row or column have been given. Dots on the border and outside the grid don't count.

What is a hidden single in Sudoku? ›

A naked single arises when there is only one possible candidate for a cell; a hidden single arises when there is only one possible cell for a candidate. Despite the name, hidden singles are far easier to find than naked singles. You should always start a Sudoku by finding all the hidden singles.

Is Sudoku for high IQ? ›

From this case study it can be concluded that an individual who is skilled at solving Sudoku puzzles likely has a high general IQ. The results of the weak correlation between Sudoku scores and the WAIT test indicates that in some cases a high Sudoku doesn't necessarily mean a high general IQ.

Does Sudoku raise IQ? ›

'Unfortunately, there's just no evidence to support that claim. 'In fact, we show that doesn't even lead to improvements to the things that are similar to the training test, let alone anything in a more general sense like IQ. 'Sleep better. Exercise regularly.

Is Sudoku good for seniors? ›

Many aging adults enjoy sudoku because it allows them to reacquaint themselves with various brain regions that they might not often use in daily life. The game improves your capacity for pattern and opportunity recognition while also teaching the mind to employ its logical and deductive thinking abilities.

What is the red number in Sudoku? ›

There are three squares which contain only three values: 4, 6, and 8. If these numbers were written in any square where they are red, it would be impossible to complete the squares where they are blue. Therefore, the numbers in red can be erased.

What is the last possible number rule in Sudoku? ›

Last possible number is a simple strategy that is suitable for beginners. It is based on finding the missing number. To find the missing number you should take a look at the numbers that are already exist in the 3x3 block you are interested in, and in the rows and columns connected with it.

What is the average time to solve a Sudoku? ›

Normally it takes 4–6 minutes, but the really hard ones sometimes take 15 minutes to be solved. My all time best time record is 2mins57secs for the extreme level on sudoku genina. How long do most people take to complete a Sudoku puzzle?

What is the best square to start Sudoku? ›

Look for the easy play first: When you first start to play a Sudoku puzzle, look for where you have the easiest opportunities to add a number. Usually this is where there is a crowded square or a row that is almost full of numbers.

What is the fewest number to solve Sudoku? ›

The fewest clues required for a proper Sudoku is 17, but not all completed grids can be reduced to a proper 17 clue Sudoku. About 49,000 unique (non-equivalent) Sudokus with 17 clues have been found. (A proper Sudoku has only one solution).

Which level of Sudoku is the hardest? ›

Techniques and Costs
LevelLowest Difficulty ScoreHighest Difficulty Score
2 more rows

Which is better for brain chess or Sudoku? ›

Generally speaking, Sudoku is considered more accessible for beginners, while Chess is considered more challenging and requires more time to master. While both are both challenging games, their level of difficulty varies. Sudoku is a logic-based number puzzle game, while Chess is a strategy game.

What is a hidden triple in Sudoku? ›

"Hidden triples" applies when three cells in a row, column, or 3x3 block contain the same three Notes. These three cells also contain other candidates, which may be removed from them.

Can you solve Sudoku with math? ›

The puzzle does not depend on the fact that the nine placeholders used are the digits from 1 to 9. Any nine symbols would serve just as well to create and solve the puzzles. In fact, mathematical thinking in the form of logical deduction is very useful in solving Sudokus.

What is the 45 rule? ›

One of the most crucial regulations to keep in mind is the 45-degree rule. This planning rule stipulates that any extension to the rear, front, or side of a property must be set within a 45-degree line drawn from the nearest edge of the neighboring windows.

What is Sudoku 4 cell rule? ›

Rules of Quadruple Sudoku

The object of the puzzle is to fill in the whole 9x9 grid with numbers 1 through 9 (one number per cell) so that each row, each column, and each of the nine 3x3 boxes contains all the nine different numbers 1 through 9. The clues are in the form of sets of four special clue-numbers.

What is the 45 degree test? ›

The 45 Degree Test is used to check extensions that are perpendicular to a window, which must be the main source of light to a 'habitable room'. If the centre of the neighbour's window lies on the extension side of both of the 45° lines (on plan and elevation), then the more detailed BRE tests are required.

What is the line of sight planning? ›

Put simply, line of sight means that employees can see the connection between their goals and the organization's goals. The benefits are many – to ensure that employee's best efforts are helping achieve the organization's goals, as well as to help them know that the work they're doing matters.

What is the 45 degree rule for overshadowing? ›

The 45 degree line on the plan drawing should be drawn diagonally back from the end of the extension towards the nearest neighbouring window. If both lines cross the centre point of the nearest neighbouring window then it is likely that overshadowing will occur.

What is the magic number in Sudoku? ›

Now Gary McGuire, a mathematician at University College Dublin, has come up with what he says is a proof that finds the minimum number of clues, or starting digits, needed to complete the game is 17.

Is Sudoku good for the brain? ›

International Sudoku Day 2022: Playing Sudoku not only improves concentration and memory but also cuts risk of developing Alzheimer's disease. Playing Sudoku can not only improve concentration and memory for children, but for elderly it can keep Alzheimer's - the most common forms of dementia - away.

What is diabolical Sudoku? ›

Classic sudoku: fit the numbers in the grid so that each row, column and square contains the numbers one to nine once only. If you think it sounds simple then try our diabolical sudoku to see how hard it can be. All Sudoku Diabolical Puzzles. No 6,089.

What game is harder than Sudoku? ›

Like sudoku, kakuro is a cross sums puzzle that requires logic and patience to crack the grid. But, unlike sudoku, kakuro incorporates filled and empty cells similar to a crossword puzzle for a more challenging, more exciting, brain-twisting experience.

What does a colon mean in Sudoku? ›

The challenge was to fill in the above snake with the digits 1 to 9, using each digit only once. The colon “:” means divide, and you must follow the standard order of operations, meaning that multiplication/division comes before addition/subtraction.

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