## 2nd Sudoku puzzle at 4th level of hardness with its Solution based on Strategies and Techniques

This is the 2nd puzzle session at fourth level of hardness that is pegged at very hard category.

As expected, this is found to be a difficult puzzle with multiple bottlenecks to overcome even at late stages.

Digit subset analysis, Cycles and digit lockdown techniques are the main advanced techniques and resources used for overcoming the bottlenecks. Solution of each step is explained fully.

We thought it time to say a few words on hardness of Sudoku puzzles entirely learned through solving the puzzles that we published here. If you so decide, you may very well skip this following section.

### A few words on hardness of Sudoku puzzles

First to state is—

There is no simple generally accepted criterion that determines the hardness level of a Sudoku puzzle as there are no well-defined hardness levels in the first place.

Then what about the hardness levels of our puzzles that we have solved and explained till now? One aspect we may say is—surely the 2nd level puzzles are more difficult than the 1st beginner level ones. In the same way, our 3rd level puzzles solved surely are more difficult than the 2nd level puzzles.

In fact, *by solving the third levels puzzles we have discovered a host of new structures that gave rise to powerful techniques.* Mind you, that is all by self-learning—by patient search for new digit patterns and creating a new technique. This lies at the heart of problem solving.

Self-learning by pattern discovery is a key to solving harder problems.

To be honest, we cannot say this first 4th level Sudoku puzzle is harder than all the 3rd level puzzles we have solved. No new technique needed to be used in solving this puzzle.

Having said that, it cannot be denied that this puzzle needed more concentration till final solution. This is a certain indicator of hardness of the puzzle. We think that this general difficulty level arises from the single criterion of NUMBER of CELLS FILLED in the puzzle game.

As you can perceive—the less is this number the more cells you have to fill with valid digits yourself and at least it would be more tedious, if not more difficult.

The minimum number of filled cells reached by the 3rd level puzzles is 26, whereas this second 4th level puzzle has just 26 cells filled. At least from this parameter it can be perceived that the puzzle is hard to solve.

### The 2nd Sudoku puzzle at 4th level of hardness

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.

You may go through the next five sections for learning **strategies and techniques for solving hard Sudoku puzzles in brief.** Or, you may skip.

Following these five concept sections, the **solution of the puzzle is explained step by step in details.**

*Please spend your time fruitfully on the game before going through the solutions.*

### Overall strategy adopted and techniques used

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.

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 population 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.

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.

A still rarer pattern is **4 cell single digit lockdown in a rectangular formation** that we have found only once. Naturally, it is a superset of the more common single digit lockdown in 2 cells and so is much more effective.

A **basic part of overall strategy** is,

Whether we search for a

breakthrough of a bottleneckor avalid cell identification, our focus usually is on thepromising 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.**

### Structure and use of a Cycle

**Form of a Cycle: **

* In a Cycle* the digits involved are locked within the few cells forming the cycles—they can't appear in any other cell in the corresponding zone (row, column or 9 cell square) 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 4 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) generally simplifying the situation and occasionally providing a breakthrough.

**Use of a cycle:**

In the example of Cycle 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.**

This is **how a new valid cell is broken through which otherwise we were not able to find out in any other way.**

In any hard Sudoku game solution, creating, analyzing and using the structure 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

**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**

*3,5 and 9 eliminated*

**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 thein corrsponding row, column and 9 cell square, youdigits that are already filledmust include the Cycles present in the three interest zones also.

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

You may also refer to our **fi****r*** st* and

*where we first explained use of a*

**second game play sessions at level 2***and*

**Cycle***.*

**DSA**### On filling up of every empty cell DS or full DS evaluation

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

Let us see this in a little detail.

For example, to evaluate the DS for cell R1C9 in our puzzle, **look at the row R1 and column C9 that have together a set of 6 unique digits 1, 3, 5, 6, 7 and 8** with three digits missing in it—2, 4 and 9. At this point, valid digit subset or DS for R1C9 is [2,4,9].

Lastly **check the third dimension of the home square,** the 9 cell top right square, for any more possible digit cancellation. All the four digits in the 9 cell home square have already been considered and so the final DS for cell R1C9 is [2,4,9].

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 are the only candidates for filling the cell.

For full DS evaluation of all empty cells, this process is to be carried out for each empty cell carefully. Slightest error at this stage will land you into grave trouble later on.

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—

- 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,
- Identify
**promising zones**to evaluate the**DS of a few cells locally**trying for a breakthrough and so reduce the full DS evaluation load later on.

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

### Single digit lockdown and its use

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

A single digit appears only in the DSs of two or three cells inside a 9 cell square—in a single 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 inside a 9 cell square, in **no cell in the row outside the square** *the digit can appear.*

And so **you can eliminate all occurrences of the locked digit from the DSs in the row outside the 9 cell square.** If you can do that, usually it would give you the much needed breakthrough. It is a very powerful structure. And same for single digit lockdown in a column inside a 9 cell square.

As an example, if DSs in R9C7 and R9C9 in row R9 and in the bottom right 9 cell square, are [1,4,8] and [1,4] and **digit 4 appears** **only in these two DSs in the 9 cell square**, you know that the **digit 4 is locked in R9 inside the bottom right 9 cell square.**

Then if the DS in R9C1 is [3,4,7], happily **delete the locked out 4 from this DS to reduce it to just [3,7].**

*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.*

Let us solve our hard Sudoku puzzle now.

### Sudoku 2nd puzzle at fourth level of hardness

We'll show the puzzle board again for convenience of understanding.

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

The **first valid cell identified is****—**R2C6 1, scan R1,R3**.** This **first fill in a stage** is **colored turquoise blue as a convention.**

**Next few valid cells and Cycles obtained** are—

R3C3 5, scan R1,R2,C1 -- R7C7 6, scan R8,R9,C9 -- Cycle (3,7) in bottom left 9 cell square R8C3, R9C3 as [1,5,8] get cancelled out from initial DS [1,3,5,7,8] in bottom left 9 cell square by scan against C3 -- R9C1 1, DSA [1,5,8] in 9 cell square scan R9, C1 -- R8C2 5, cancel 8 from DSA [5,8] in 9 cell square scan R8 -- R7C2 8 exclusion.

By cancellation of [2,6,7] scan of R3,C4 and R3,C5 from initial DS [2,4,8,6,7] of R3C4, R3C5 in 9 cell home square, a Cycle (4,8) is formed in these two cells of R3. As a result—

R3C6 2, [6,7] cancelled in DS [2,6,7] scan R3 -- Cycle (6,7) formed in R1C4 and R1C6 -- R3C1 3, digit 9 cancelled from DS [3,9] in C3 -- R3C8 9 exclusion -- R1C2 9 cross-scan R2,C1.

As a strategy, *we don't delay in identifying a Cycle and reaping its benefits immediately.*

As a result two cycles identified for use consecutively and that resulted in a series of valid cells—

Cycle (6,7) in R1,C4 and R1,C6, an older Cycle -- Cycle (2,4) in R1C7, R1C9 of top right 9 cell square by scan on C7 and C9 respectively cancelling 8 from DS [2,4,8] in both cases -- So,

R1C1 8by exclusion -- R2C8 8 scan C7 -- R2C7 3 exclusion in 9 cell square.

At this point, the stage is closed just to take a break. The easy valid cell finds we'll show in the next stage.

The results are shown below with valid cells found **at this stage colored green.** We'll color the valid cells in the next stage in a different color to distinguish the cells filled in various stages.

#### Stage 3 of Sudoku puzzle solution: 4th level game play 2

A Cycle (2,3,4) formed in R8C7, R8C8, R9C8 and valid cell R8C9 1 by cancellation because of Cycle. This is the first valid cell in this stage.

Next steps are a bit heavy going with careful identification of possible cycles and cross-scans to get valid cells

Cycle (2,4) in C9 -- R4C9 3 -- Cycle (5,9) in R7C6, R7C9 -- R7C5 1 by cancellation of 5 -- R7C4 3 by cancellation -- R6C4 1 scan R5,C5,C6 -- R4C2 1 scan R5,R6,C1 -- Cycle (4,7,8) in R3C5, R5C5 and R6C5 -- R4C5 5 by exclusion.

Two possibilities now—Cycle (4,6,7,9) in R5 (as Cycle (2,4) in R1C7 and R8C7; Cycle (7,9) in R4C7 and R5C7) and or digit 2 locked in R6C8, R6C9 in R6. Both would result in R5C3 2. We'll take this action in next stage. The results obtained till now are shown below.

#### Stage 4 of Sudoku puzzle solution: 4th level game play 2

As analyzed in the last stage—

Due to digit 2 locked in R6C8 and R6C9 of right middle 9 cell square, 2 cancelled in R6C2 --

R6C2 4-- Cycle (6,7) in R4C1, R6C1 by cancellation of 4 -- R6C3 9 by cancellation --R5C3 2 by exclusion-- R2C2 2 by exclusion in C2 -- R2C3 6 by cancellation --R2C1 4 exclusion in 9 cell square.

Now turn attention to the central region**—**

R6C5 8 scan R4,R5 -- R3C5 4 by cancellation -- R3C4 8 by exclusion -- R5C5 7 exclusion -- R5C7 9 cancellation --

R4C7 7 by cancellation-- R4C1 6 by cancellation -- R6C1 7 exclusion -- R6C9 2 by cancellation -- R6C8 6 exclusion --R1C9 4 cancellation-- R1C7 2 exclusion in 9 cell square -- R8C7 4 exclusion -- R9C8 3 cancellation -- R8C8 2 cancellation -- R4C8 4 exclusion --R8C4 7 cancellation.

And the last valid cells—

R8C3 3 exclusion -- R9C3 7 exclusion -- R1C4 6 cancellation -- R5C4 4 cancellation --

R9C4 9 exclusion-- R9C9 5 cancellation -- R9C6 4 exclusion -- R5C6 6 cancellation -- R7C9 9 exclusion in C9 -- R7C6 5 exclusion -- R4C6 9 exclusion -- R1C6 7 exclusion.

The final solved puzzle board is shown below.

Check for the validity of the solution if you need.

### End note on Problem solving in Sudoku

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

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

As this Sudoku problem is large and complex, we had to repeatedly stop at step 3 to collect detailed DS information of the cells of interest, and not for all the empty cells. Effectively then the last 5 steps have been repeated. DS information populated in stages.

Key pattern identification had occurred in many instances primarily by Cycle formation and use and once by digit lockdown.

Because of the large number of empty cells, chance of making error was high.

Watch out for the next 4th level Sudoku puzzle solution.

### Other Sudoku game plays at fourth level hardness

**Hard fourth level Sudoku puzzle 6, Strategies Techniques and Solution**

**Hard fourth level Sudoku puzzle 5, Strategies Techniques and Solution**

**Hard fourth level Sudoku puzzle 4, Strategies Techniques and Solution**

**Hard fourth level Sudoku puzzle 3, Strategies Techniques and Solution**

**Hard fourth level Sudoku puzzle 2, Strategies Techniques and Solution**

**Hard fourth level Sudoku puzzle 1, Strategies Techniques and Solution**

### Assorted Interesting Sudoku game plays

These Sudoku game solutions are collected from various sources and are found to be interesting. You can get these Sudoku solutions at * Interesting Sudoku *not classified at any hardness difficulty level.

### First and second level Sudoku games

You will get the earlier Sudoku game solutions at * Beginner level Sudoku* and

**Second level Sudoku.**### Third level game plays

List of third level hard Sudoku game plays are available at **Third level Sudoku.**