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Very Hard Sudoku Expert Level 5 Game 8 Easy Solution

Very Hard Sudoku Expert Level 5 Game 8 Easy Solution

Sudoku Strategy and Techniques: Very hard Sudoku Expert level 5 game 8 Easy solution

The very hard Sudoku Expert level 5 puzzle game 8 solved using strategy and techniques that ensured natural way to the solution with minimum waste of time.

Sections are,

  1. Sudoku hard level 5 game 8
  2. Solving Very Hard Sudoku Expert level 5 game 8 by Sudoku hard strategy and techniques
  3. Sudoku hard Strategy and techniques for easy solution
    1. What is a Cycle and how to use it in solving a Sudoku hard puzzle. Frequently used technique.
    2. How a single digit candidate valid cell is identified by Digit Subset Analysis (DSA) in solving a Sudoku hard puzzle. Heavily used technique.
    3. How digits possible for all empty cells (DSs) enumerated while solving a Sudoku hard puzzle. To be used judiciously, better to use partially.
    4. Single digit lockdown and its use in solving a Sudoku hard puzzle. Helps to make a breakthrough in a Sudoku hard puzzle.
    5. Sudoku hard technique of double digit scan. Helps to make a breakthrough in a Sudoku hard puzzle by creating new digit pattern like a Cycle.
    6. Sudoku hard expert level breakthrough technique of X wing or single digit rectangle. Helps to make a critical breakthrough in a Sudoku hard puzzle.
    7. Sudoku technique of parallel scan for a single digit on a row or a column: For a breakthrough in hard Sudoku puzzles.

You may move directly to any of the above sections by clicking its link and return to previous position by clicking on browser back button.

While going through the solution you may click on say Cycle whenever it appears, to know how to form and use a Cycle and then return to the previous position to continue through the solution.

In the same way, for the other techniques also you may jump to details of a technique and after refreshing your concept return to the point from where you jumped.

Very hard Sudoku expert level 5 game 8

The following is the Sudoku puzzle that should engage your mind for some time. The Rs are the row labels, Cs are the column labels and this we define as the stage 1 marked on top left corner.


We'll first solve the Sudoku hard using strategy and techniques for solving Sudoku hard puzzles.

The strategy and techniques for quickly solving Sudoku hard are explained with examples in five sections after the solution.

You may decide to go through the solution directly and take a look at the specific strategy or technique when you feel like.

The Sudoku hard techniques with internal links to the concept sections appear in the solution. You may just click on any such link, say for example Cycle, go through the concept and return by clicking on browser back button.

Or, you may decide first to go through the concepts and then go through the solution. To jump to the Strategy and techniques concept sections click here.

Following is the solution of the puzzle explained step by step in details.

Please spend your time fruitfully on the game trying to solve it before going through the solutions.

Solution to the Very hard Sudoku Expert level 5 game 8 Stage 1: Breakthrough by Parallel digit scan, Double digit scan, X wing and Cycle of twins

Let us solve our Expert level 5 Sudoku hard puzzle now.

We'll show the puzzle again for ease of understanding.

To follow the details accurately, you should better have the game actually with you written on paper, or better stillcreated in a spreadsheet.


Strategy adopted in brief: start with row column scan coupled with aggressive breakthroughs by advanced Sudoku techniques.

To skip the rest of the brief strategy click here.

As a strategy we always start row column scan from digit 1 and continue till digit 9. In the process we may get valid cells, but otherwise our lookout is for identifying a single digit lock by cross scan.

If we get one, we should get usually a breakthrough in one step or two.

In addition, at this early stage while carrying on row column scan, we remain alert for the possibility of a double digit scan or even a parallel scan also.

When possible valid cell by a row column scan is exhausted, we take up the next two patterns of possible digit analysis DSA technique and forming Cycles of twins or triplets (or of even larger number of cells).

A little later when more cells are filled with unique digits, we raise our alertness for more powerful breakthrough digit patterns such as an X wing.

On the whole, we are always ready to force a breakthrough using even an advanced Sudoku technique at the early stages if feasible.

On the other hand, we are always ready to look for the simplest method of row column scan even when facing a bottleneck.

Main focus is to either find a valid cell or forming a breakthrough pattern such as Single digit lock, Cycles of twins or triplets or X wing at the earliest.

In the process, avoiding enumeration of time-consuming and unnecessary evaluation of possible digit subsets in empty cells speeds up the solution considerably.

Highlight color of valid cells in this first stage is banana leaf green.

The highlight colors for cells involved in a Cycle of twins, triplets or quadruplets are primarily yellow and for two adjacent Cycles light orange.

The term names are all ours as we learned how to solve Sudoku and created all the term names (for labels to techniques) by actually solving easy to extremely hard Sudoku games. But that should be easy to relate to as we have kept the names FUNCTIONAL AND MEANINGFUL.

Only recently when referring to some Sudoku solution literature we liked a few term names and included it in our terminology.

First valid cells by row column scan:

R4C5 1 by scan for 1 in C4, C6 on empty cells of central middle major square.

R6C1 4 by scan for 4 in R4, R5, C3 -- R7C2 4 by scan in C1, C3.

R2C3 7 by scan for 7 in R1, C1, C2 -- R3C4 7 by scan for 7 in R1, R2.

R9C1 8 by scan for 8 in C3 -- R9C3 1 by scan for 1 in R7, R8.

Breakthrough by parallel digit scan for 1 on empty cells of R6:

R6C3 eliminated for 1 by 1 in C3, R6C4, R6C6 eliminated by 1 in central middle major square and R6C9 eliminated by 1 in C9 -- R6C7 1.

It follows R2C8 1 by scan for 1 in C7, C9.

Breakthrough Cycle of twins by double digit scan:

Cycle of twins (4,9) formed in R1C7, R2C7 by single row double digit scan for [4,9] in R3. Another Cycle of twins (3,6) formed in R3C7, R3C9 by the leftover two digits in top right major square.

Possible digit subset in R3 reduced to [2,5] and with 2 in C2, R3C2 5 by reduction -- R3C1 2 by exception in R3.

Cycle (5,6) of twins formed in R7C3, R8C3 by reduction of existing digit subset [2,3,9] in bottom left major square from possible digit subset [2,3,5,6,9] in C3.

Next breakthrough is again by parallel digit scan:

Further 3 in R5 and 3 in R6 reduces the reduced DS [2,3,9] to Cycle of twins (2,9) and valid cell breakthrough by reduction R1C3 3. This is equivalent to a parallel digit scan for 3 on empty cells of R3.

It follows R4C2 3 by scan for 3 in R5, C1, C3.

Now a breakthrough by forming a series of Cycles of twins and triplets by DSA reduction:

Possible digit subset [3,6] enumerated in R9C9 by reducing [2,8,9] in R9 from DS [2,3,6,8,9] in C9. This joins with [3,6] in R3C9 to form a new Cycle of twins (3,6) in C9 and another Cycle of triplets (2,8,9) in R5C9, R6C9 and R8C9.

As a result the DS [2,9] of R6C9 joined with [2,9] in R6C3 to form a new Cycle of twins (2,9) in R6. So it forms Cycle of twins (5,6) in R6C4, R6C6 and also with 9 in C6 another Cycle of twins (2,7) in R4C6, R5C6 in central middle square and breakthrough valid cell R4C4 9.

Check for yourself.

Breakthrough Sudoku digit pattern X wing formed:

Single digit lock on 2 formed in R5C9, R6C9 by scan for 2 in C7, C8 and this lock joins with already existing lock on 2 by the Cycle of twins in R5C3, R6C3 to create the powerful digit pattern of an X wing. These two pairs of digit 2 in possible digit subsets share the rows R5, R6 as well as columns C3, C9.

As each of these incidences of digit 2 are only two possible incidences in their respective columns one of these two in each column must be there in the final solution. It means, final solution must have digit 2 at the two cells at the end of one of the two diagonals thus blocking both pairs of rows and columns for 2.

As a result, we get the critical breakthrough R5C6 7 by reduction of [2] from DS [2,7] followed by R4C6 2.

You may verify the actions taken till now from the first stage status below.


Solution to the very hard Sudoku expert level 5 game 8 Stage 2: Breakthrough by X wing, DSA reduction, Cycle of twins

Highlight color of valid cells at this second stage is light blue.

We have ended the last stage with a breakthrough by an X wing and in the same way we will start the second stage with another critical breakthrough by a second X wing:

The second X wing is formed on 5 in cells R1C4, R1C6, R6C4, R6C6 with both pairs of 5s sharing common rows and columns. This reduces digit 5 from DSs R7C4, R9C4, R8C6 and R9C6.

Way to the major breakthrough by DSA reduction and Cycle formation:

By existing digit subset reduction of [4,7,5] in C4 from DS [3,4,5,6,7] in R9, DS in R9C4 reduces to [3,6] and joins with [3,6] in R9C9 to form the breakthrough Cycle of twins (3,6) in R9 -- R9C6 4 by reduction of [5,7] (5 by X wing on 5) from row DS of [4,5,7] -- R8C5 5 by reduction -- R9C5 7 by reduction -- R7C5 2 by reduction -- R9C8 5 by exception as only digit left in R9 apart from the Cycle of twins.

R8C8 4 by scan for 4 in R7, lock on 4 by Cycle in C7, 4 in C9.

R2C5 4 as exception in C5.

Last breakthrough at this stage is by parallel digit scan:

R5C8 9 by parallel scan for 9 on empty cells of C8 by 9 in R4 and R7 leaving only the cell R5C8 for 9.

Lots of breakthroughs in these first two stages ensuring a quick and easy solution to this very hard Sudoku expert puzzle.

Second stage game status below.


Solution to the very hard Sudoku Expert level 5 game 8 Stage 3: Breakthrough by Parallel digit scan

Highlight color of valid cells at this third stage, which is not the last stage, is light grayish yellow.

With 9 in R5C8, a pending valid cell R6C9 2 -- R5C9 8 by reduction -- R8C9 9 by reduction.

With 2 in R6C9, R6C3 9 by reduction -- R5C3 2 by reduction.

With 5 in R8C5, pending valid cell R8C3 6 -- R7C3 5 by reduction.

This is followed by a breakthrough valid cell by parallel scan:

R2C4 2 by parallel digit scan for 2 on empty cells of C4 by elimination of R9C4, R7C4, R6C4 and R1C4 for 2 by 2 in R9, R7, R6 and R1 leaving only R2C4 for 2.

It follows, R9C4 3 as the only cell in C4 in which 3 can appear -- R9C9 6 by reduction -- R7C8 7 by reduction -- R7C7 8 by reduction -- R8C7 3 by reduction.

We'll stop here not willing to load this stage further. Rest of the valid cells are just a formality.

The status results are shown below.


Solution to the very hard Sudoku expert level 5 game 8 Final stage 4: Rest are easy valid cells

Highlight color of valid cells at this fourth and last stage is light cyan.

With 3 in R8C7, R3C7 6 by reduction -- R3C9 3, R5C7 5 by reduction of 6 -- R4C7 7 by reduction of [5,6] -- R4C8 6 by reduction of 7 -- R4C1 5 by reduction.

With 8 in R7C7, pending valid cell R7C4 6 by reduction -- R8C6 8, R6C4 5 by reduction of 6 -- R4C6 6, R1C4 8 by reduction of 5 -- R1C6 5, R2C6 3, R1C2 1 by reduction of 8 -- R1C1 9 by reduction of 1 -- R2C1 6 by reduction of 9 -- R2C2 8, R5C1 1 by reduction of 6 -- R5C2 6 by reduction of 1.


The final solved puzzle is shown below.


Check for the validity of the solution if you need.

Overall strategy adopted and techniques used for solving a Sudoku hard puzzle

As a strategy we always try first—the row-column scan to find the valid cell at any stage, because that is the most basic and easiest of all techniques.

When easy breaks by row-column scan becomes hard to come by, the next technique is used.

Next easy to use technique used is—identification of single valid digit for a cell by Digit Subset Analysis or DSA in short. This technique is explained in a following concept section.

And wherever possible, Cycles are formed that in any situation are a treasure to have and Cycles play a key role in quick solution. Concept and use of Cycles are explained in a following section.

You may wait for Cycles to form automatically in a column or row.

But a proactive approach of forming a Cycle by DS analysis speeds up the solution process considerably. This is what we call forced creation of Cycles.

The last resort of filling EACH EMPTY CELL with valid digit subsets is to be taken when it is absolutely necessary. Only with all empty cells filled with valid digit subsets, the possible breakthrough points in a hard puzzle can be discovered.

Strategically for faster solution, it is better to delay this time consuming task as much as possible.

Full DS enumeration process is explained in a following section, but any experienced Sudoku player would be doing it as a routine.

In hybrid strategy, a few of the cells of interest are filled with DS of shorter length and analyzed for a breakthrough such as forming a Cycle or a single digit lockdown.

One of the most powerful patterns that we have used for highly positive result each time is the lockdown of a single digit in a row or column inside a 9 cell square so that the digit is eliminated from all other DSs in the locked row or column outside the 9 cell square.

The necessity of use of this digit lockdown technique indicates in a way the hardness of the puzzle. This technique is also explained in a following section.

In solving this expert level very hard Sudoku puzzle, a number of breakthroughs are by Parallel scan for a single digit on the cells of a row or column. This is an extremely powerful technique if it can be spotted easily or at all.

A rarely encountered powerful pattern is 4 cell single digit lockdown in a rectangular formation that may be encountered in very hard or expert level Sudoku hard puzzles. Naturally, it is a superset of the more common single digit lockdown in 2 cells and so is much more effective.

This powerful digit pattern is the single digit rectangle or X wing. In solving this Expert level 5 Sudoku hard puzzle, X wing provided the most important breakthrough.

A basic part of overall strategy is,

Whether we search for a breakthrough of a bottleneck or a valid cell identification, our focus usually is on the promising zones, the zones (row, column and 9 cell square combined) that contain larger number of filled digits including Cycles.

The main strategy should always be to adopt the easier and faster technique and path to the solution by looking for key patterns all the time. Digit lockdown, Cycles, Valid cell by DSA are some of the key patterns.

Focus when solving a hard Sudoku puzzle should be on using the technique that would produce best results fastest. Easy to say, not so easy to do—comes with practice.

What is a Cycle and how to use it in solving a Sudoku hard puzzle

Form of a Cycle:

In a Cycle, the digits involved are locked within the few cells forming the cycle. The locked digits can't appear in any other cell in the corresponding zone outside the few cells forming the cycle.

For example, if a 3 digit cycle (4,7,8) in column C2 is formed with a breakup of, (4,7) in R1C2, (4,7,8) in R5C2 and (7,8) in R6C2, the digits 4, 7 and 8 can't appear in any of the vacant cells in column C2 further.

If we assume 4 in R1C2, you will find R5C2 and R6C2 both to have DSs (7,8) implying either digit 7, or 8 and no other digit to occupy the two cells. This in fact is a two digit cycle in the two cells. Together with 4 in R1C2, the situation conforms to only digits 4, 7 and 8 occupying the set of three cells involved in the cycle.

Alternately if we assume 7 in R1C2 (this cell has only these two possible digit occupancy), by Digit Subset cancellation we get, digit 8 in R6C2 and digit 7 in R5C2 in that order repeating the same situation of only the digits 4,7 and 8 to occupy the set of three cells.

Effectively, the three digits involved cycle within the three cells and can't appear outside this set of three cells.

This property of a cycle limits the occupancy the cycled digits in other cells of the zone involved (which may be a row, a column or a 9 cell square). This generally simplifies the situation and occasionally provides a breakthrough by reducing the number of possible digits in the affected cells.

A number of Cycles are shown below from a Sudoku hard solution stage:

Examples of Cycles in a Sudoku game

Cycle (1,2,6) in column C1 is over all three 9 cell squares on the left. It affects only the column C1.

Cycle (3,8,9) in top right 9 cell square is also in row R2, so it should affect both the 9 cell square and R2.

But Cycle (3,6,7) in top right 9 cell square is formed only in the 9 cell squares, it affects only the cells in the 9 cell square.

Can you see another Cycle in row R1 apart from Cycle (1,6)? The second Cycle (3,6,7) is formed by the cells R1C2, R1C3 and the far away cell R1C9. This Cycle affects only the row R1.

Can you say which are the affected areas for Cycle (1,6) in R1?

Two cells of this Cycle belong to row R1 as well as to the top middle 9 cell square. So the Cycle affects two areas, the row and the 9 cell square. This will be true for any two digit Cycle.

Use of a cycle:

In the example of cycle described above, if a vacant cell R8C2 in column C2 has a possible DS of (1,4), as digit 4 has already been consumed in the cycle (4,7,8) in the column, only digit 1 can now be placed in R8C2. You get a single valid digit 1 for R8C2.

This is how a new valid cell is obtained using a Cycle that was not visible otherwise.

In any hard Sudoku game solution, creating, analyzing and using the pattern of Cycles play a very important role.

How a valid cell is identified by Digit Subset Analysis or DSA in short

Sometimes when we analyze the DSs in a cell, especially in highly occupied zones with small number of vacant cells, we find only one digit possible for placement in the cell. We call valid cell identification in this way as Digit Subset Analysis.

For example, if in row R4 we have four empty cells, R4C1, R4C3, R4C6 and R4C9 with digits left to be filled up [1,3,5,9] we say, the row R4 has a DS of [1,3,5,9] that can be analyzed for validity in each of the four empty cells.

By the occurrence of digits in other cells if we find in only cell R4C1 all the other three digits 3,5 and 9 eliminated as these are already present in the interacting zones of middle left 9 cell square and the column C1, we can say with confidence that only the left out digit 1 of the DS [1,3,5,9] can occupy the cell R4C1.

Remember that,

While evaluating the valid digit subset or DS of an empty cell, you would analyze not only the digits that are already filled in corresponding row, column and 9 cell square, you must include the Cycles present in the three interest zones also.

This is how we identify a valid cell by Digit Subset Analysis.

Identifying a valid digit in a cell by DSA is like a bread and butter technique. It is possibly the most heavily used technique after the simplest row-column scan.

Though DSA may not be considered as an advanced technique it often provides a much required breakthrough. So always look for a valid cell by DSA.

An example of a breakthrough at the late stage of Sudoku hard puzzle solution by DSA is shown below.


We'll do DSA on cell R7C5. The possible digit subset or DS in column C5 and hence in cell R7C5 is [5,7,9], but the two digits [5,9] both are present in row R7.

So eliminating these two from the three digit DS for R7C5, we get the single valid digit 7 for R7C5 --- R7C5 7.

This is a breakthrough even at this late stage.

How digits possible for all empty cells (DSs) enumerated while solving a Sudoku hard puzzle

We have not yet discussed the enumeration of every empty cell with their valid digit subsets or DSs.

Let us see this in a little detail. We'll enumerate the possible digit subset or DS for empty cell R8C1 in the following Sudoku game.

Possible digit subset DS enumeration for an empty Sudoku cell

Target cell R8C1 is colored green. Unique set of digits in the three zones—bottom left 9 cell square, row R8 and column C1 colored yellow—will determine the DS for empty cell R8C1.

To enumerate the DS for cell R8C1, look at the row R8 with six digits missing in it—1, 2, 4, 5, 6 and 7.

Now cross-scan column C1 to identify any of these six appearing in column C1.

As 5 and 7 are the two digits out of six that are missing in the intersecting row R8, cancel these two from the six digit subset for R8C1 to reduce it to [1,2,4,6]. Considering row R8 and column C1, possible digits that can occupy R8C1 till now are the DS [1,2,4,6].

But R8C1 also belongs to a 9 cell square and filled digits in it will affect the DS for the cell.

So lastly check the third dimension of the home square, the 9 cell bottom left square, for any more possible digit cancellation.

With no additional digit cancellation, the valid digit subset or DS for the cell would be four digits [1,2,4,6].

None of these four digits appear in the home square, home column or the home row for the cell R8C1.

Basically for evaluating the valid DS for a cell,

You have to cross-scan the row and column as well as check against the home square digits to identify the missing digits that may fill the cell.

This is a tedious and error-prone process.

In solving a hard Sudoku puzzle, there may be no option than to go through the full empty cell DS evaluation.

But it should be done when it has to be done and as late as possible.

Two strategic approaches are adopted to minimize the overall work load in this process—

  1. First try to find valid digits and fill the cells as much as possible using any technique so that the number of possible valid digits in empty cells as well as number of empty cells are reduced, and,
  2. Identify promising zones to evaluate the small DSs of a few cells trying for a breakthrough and so reduce the full DS evaluation load.

The second is a dynamic approach that depends on your experience and skill in identifying promising zones.

Single digit lockdown and its use in solving a Sudoku hard puzzle

Occasionally, after evaluating valid DSs for a number of empty cells, you may find that,

A single digit appears only in the DSs of two or three cells in a 9 cell square, in a column or a row, and in no other DSs in the 9 cell square.

This is what we call as single digit lockdown.

If it happens in a row (or a column) inside a 9 cell square, the digit cannot appear in any other cell in the row (or the column) outside the square.

This eliminates all occurrences of the locked digit from the DSs in the row (or the column) outside the 9 cell square. Usually it creates a much needed breakthrough. It is a very powerful pattern.

Single digit lockdown - Conditions for single digit lockdown - how to identify it

Two conditions for single digit lockdown pattern,

  1. the digit can be placed in only two or three cells of a column or a row, AND,
  2. the locking cells must also be in SAME 9 cell square.

The third desired condition is,

  • The lockdown to be effective, the locked digit should not be present as a single cell candidate in both the adjacent two 9 cell squares through which the locked column or row passes.

The following shows an example of single digit lockdown of 5 in cells R7C1 and R9C1.

sudoku single digit lock: what it is and what it does

How a single digit lockdown is formed

Look at columns C1, C2 and C3 in the bottom left 9 cell square R7R8R9-C1C2C3. Out of 3 empty cells, the cell R7C3 is debarred for placing digit 5 as column C3 has a 5 and it lights up the cell for digit 5.

5 can appear only in two cells in column C1, R7C1 and R9C1 and in no other cell in the 9 cell square or the column C1.

It is locked inside these two cells in C1 and 9 cell parent square.

How a Sudoku single digit lockdown is used - What it does

The locked digit 5 eliminates itself from the DSs of the other two empty cells R5C1 and R6C1 and a new Cycle (2,3) is created in C1.

Focus again on the bottom left 9 cell square. With Cycle (2,3) in C1, another Cycle (5,9) is formed in the two cells of the 9 cell square. As a result, digit 1 becomes the only digit left and cell R7C3 only cell left for it in the 9 cell square.

Still more happens. With 1 in C3 now, digit 9 now must occupy the cell R6C3.

These two single digit candidates obtained by the single digit lockdown of 5 affects other cells and breaks the bottleneck.

As a strategy, always form a single digit lock as soon as it is discovered.

You may think, what is the point of it, what would it achieve after all!

Well, in a similar situation in the process of solving a hard Sudoku puzzle game, the reduced DS in R9C1 formed a cycle (3,7) in column C1 and helped to pinpoint a valid digit 4 in cell R2C1 and that started a deluge of valid cell finds. This proved to be the key turning point in the whole game.

Sudoku hard technique of Double digit scan

This technique sounds simple, but being aware of its existence and identifying it would always result in an important breakthrough. This digit pattern usually occurs in very hard Sudoku.

We will explain this advanced Sudoku hard technique on the following situation in a Sudoku hard game,

Sudoku hard technique of double digit scan

Notice the two highlighted digits [1,6] appearing in both row R4 and C5. Together these two result in DIRECT FORMATION OF CYCLE (1,6) in central middle 9 cell square.

This is a double digit scan simultaneously on a row and a column.

Now observe a second set of highlighted double digits [3,9] in C5 which DIRECTLY FORMS TWO CYCLES (4,7,8) AND (3,9) IN CENTRAL MIDDLE 9 CELL SQUARE.

This is a double digit scan on a single zone of C5.

Finally, with 3 in C4, R4C4 9 and R4C6 3.

Together these two double digit scans have produced two valid cells and two Cycles. It is a major breakthrough early in the Sudoku hard game.

Sudoku hard expert level breakthrough technique of X wing

This special and not often occurring digit pattern involves A SINGLE DIGIT SHARING SAME TWO ROWS AND TWO COLUMNS.

This can be imagined as an advanced form of single digit lock over one pair of rows and one pair of columns in the formation of a rectangle. That's why we call it a single digit rectangle.

This digit formation is commonly known as X wing.

The following figure shows an X wing formation on digit 4 at an advanced stage of solution.

Sudoku hard expert level breakthrough technique of X wing or single digit rectangle

At this advanced stage, possible digit subsets for all empty cells are evaluated after the valid cells and easily formed Cycles are created. By this approach, time waste is minimal.

Interestingly, if you examine closely you will find that all the possible digit subsets are involved in one or more than one Cycle.

The second more important aspect of this puzzle status is,

You cannot find any more valid cell breakthrough by the more frequently used techniques of row column scan (hidden singles), DSA (naked singles), Cycles (naked and hidden groups), single digit lock (direct interaction), double digit scan or parallel scan.

In this situation especially, you need to identify the more complex breakthrough digit patterns of X wing, XY wing or Swordfish. Specialty of all these next advanced level digit patterns is,

Usually all of X wing, XY wing or Swordfish digit patterns involve 4 or more cells with two possible digits and one common digit.

Observe that we have indeed such a digit pattern formation in four cells R3C3, R3C8, R6C8 and R6C3.

The common digit is 4 in all these four two digit long possible digit subsets. And this single digit of 4 is shared between two columns C3, C8 and two rows R3, R6.

What's so special about this formation?

The special effect of the X wing formation is simply,

The commonly shared digit cannot appear in any cell of these rows and columns expect the four cells. As a result all instances of commonly shared digit in these other cells are eliminated, and this REDUCTION INVARIABLY PRODUCES A CRITICAL BREAKTHROUGH.

In our example game, the X wing cause reduction of 4 in possible digit subset [4,9] in cell R6C3 and causes the most critical breakthrough of the puzzle in the form of,

R7C3 9.

In fact, with this valid cell breakthrough, rest of the cells are all reduced to valid cells just by reduction.

Recommendation: If at any stage of the game you find many two digit long possible digit subsets with one common digit in many, look for X wing.

Sudoku hard technique of Parallel scan

A parallel scan is carried out for a specific digit on the empty cells of a promising row (or column). Because of presence of the specific digit in the interconnecting columns (or rows) for all empty cells of the scanned row (or column) except one, the valid cell for scanned digit can be identified as this cell.

The digit pattern and the technique to identify a breakthrough valid cell by parallel scan is shown in the figure below,

Sudoku hard level 4 technique of parallel scan

The parallel scan for digit 6 is done in this case on the empty cells of R1. Out of 4 empty cells R1C4, R1C6, R1C7 and R1C9, digit 6 is disallowed in the first two by 6 in top middle major square and disallowed in R1C7 by 6 in C7.

This leaves only the single cell R1C9 where digit 6 can be placed. That becomes the valid cell for digit 6.

Observe that as a result a Cycle (4,7,9) is formed in the rest of the three empty cells in R1.

If you could have identified the Cycle before parallel scan, you could automatically have got the valid cell without parallel scan. That's the interesting property of parallel scan, if you can spot one, you would be sure to find an equivalent Cycle as a result.

To us, valid cell breakthrough by parallel scan is easier and faster.

To go through the solution of this Sudoku hard once more, click here.

End note on Problem solving in Sudoku

Any puzzle solving involves essentially problem solving. The general steps are,

  1. First stage analysis and breaking it down into smaller chunks if possible as well as adapt the strategy of solving this type of problem,
  2. Solving the easier component problems so that the main problem size and complexity is reduced,
  3. Detailed information collection, that is, defining the problem in more details as far as possible,
  4. Second stage analysis of structure of problem (in this case of Sudoku) and information content,
  5. Key pattern identification,
  6. Use of the key pattern to create the breakthrough,
  7. Repeating the last five steps (steps 3, 4, 5, 6 and 7) for finally solving the problem.

As a strategy, focus has been to form advanced breakthrough digit patterns at the earliest by evaluating as few promising cells as possible. This speeds up the solution as well as breaks down the barriers quickly.

Key pattern identification had occurred in many instances by Cycles, DSA, Double digit simultaneous scan, parallel scan, single digit lock and X wing.

Critical breakthroughs have been provided by X wing formation of a single digit shared between four cells in two columns and two rows. Use of this powerful technique in the first stage as well as early in the second stage for major breakthroughs places the Sudoku hard game surely as a very hard Sudoku expert puzzle.

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