Mirrored from **Sudopedia**, the Free Sudoku Reference Guide

The **Uniqueness Tests** are a set of techniques that avoid the various deadly patterns, assuming that the given Sudoku is valid. The patterns involved are often easy to recognize, but it should be noted that since a deadly pattern will never occur in a valid Sudoku, these techniques are not strictly necessary to avoid them -- the puzzle can always be solved by other means. Or so it is claimed--see Uniqueness Controversy.

This article focuses on the uniqueness tests used to avoid the Unique Rectangle deadly pattern. The techniques used to avoid the Bivalue Universal Grave (BUG) and BUG Lite patterns are discussed in the BUG and BUG Lite articles. See also the Avoidable Rectangle article.

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Generally, the Unique Rectangle tests consider four cells that form a rectangle whose common candidates are two digits, with one or more of these cells containing extra candidates as well. The 4 cells in the corners of the rectangle belong to exactly 2 rows, 2 columns and 2 boxes. To avoid this deadly pattern, at least one of the extra candidates must be placed.

In many of the patterns for the Uniqueness Tests, two cells on one side of the rectangle (i.e., sharing either the same row or the same column) have only the common two candidate digits and no extra candidates. These cells form the **floor** of the rectangle. The other two cells are called the **ceiling**.

Uniqueness Test 1 is the simplest among all uniqueness tests. It can be stated as follows:

- If there is only one cell in the rectangle that contains extra candidates, then the common candidates can be eliminated from that cell. A rectangle that meets this test is also known as a Type 1 Unique Rectangle.

An example below:

Suppose **r9c2** contains only either 3 or 6, i.e. having 3 and 6 as its only candidates. Then the four cells **r7c2**, **r7c4**, **r9c4** and **r9c2** forms a Unique Rectangle deadly pattern on the digits 3 and 6, which makes the puzzle invalid. Therefore, both 3 and 6 can be eliminated from **r9c2**.

Possible Eureka notation:

(9)r9c2 = (36)UR:r79c24 => r9c2 = 9

As the UR must be false, (9)r9c2 must be true

This test can be stated as follows:

- Suppose both ceiling cells in the rectangle have exactly one extra candidate
**X**. Then**X**can be eliminated from the cells seen by both of these cells. A rectangle that meets this test is also known as a Type 2 Unique Rectangle.

Note the similarity with Uniqueness Test 5 described later.

Uniqueness Test 2 can be illustrated in the example below:

**r5c4** and **r5c6** form the floor of the rectangle with **r2c4** and **r2c6** as the ceiling. To avoid the Unique Rectangle for the digits 8 and 9, either **r2c4** or **r2c6** must contain the digit 7. Therefore, 7 can be eliminated from any cell that is seen by both **r2c4** and **r2c6**.

Possible Eureka notation:

(8=7)r2c5 - (7)r2c46 = (89)UR:r25c45 => r2c5 = 8

As before, because the UR must be false, (8)r2c5 must be true

This test can be stated as follows:

- Suppose both ceiling cells have extra candidates. By treating these two cells as one node, find
*k*- 1 other cells (as nodes) in the same house as these two cells so that the union of the candidates for these*k*cells has exactly*k*unique digits. Then the Naked Subset rule can be applied eliminate these*k*digits from the other cells in the house. A rectangle that meets this test is also called a Type 3 Unique Rectangle.

In a way, Uniqueness Test 3 can be seen as an extension of Uniqueness Test 2. When *k* = 1, Test 3 reduces to Test 2.

An example below, from the Players' forum:

**r2c5** and **r2c6** are the floor of the rectangle with **r7c5** and **r7c6** being the ceiling. By treating **r7c56**, **r8c4** and **r9c4** as a Naked Triple on the digits 2, 6, and 9, we can eliminate 6 from **r8c6** and 2 from **r9c5**.

Possible Eureka notation:

(6=92)ALS:r89c4 - (2)r7c5 =[(57)UR:r27c56]= (6)r7c6 - Loop => r8c6 <> 6, r9c5 <> 5

(2)r7c5 and (6)r7c6 can't both be false otherwise the UR is formed. There is therefore a strong inference between them. All the links in a closed loop are shown to be conjugate which provides the exclusions. Alternatively:

(6=92)ALS:r89c4 - (2=57)ALS:r27c4 -[UR]- (57=6)ALS:r27c5 - Loop => r8c6 <> 6, r9c5 <> 5

Here the digit pair 57 is considered to be weakly linked between the UR cells in the two columns.

This test can be stated as follows:

- Suppose both cells in the ceiling contain extra candidates. Suppose the common candidates are
**U**and**V**, and none of the cells seen by both ceiling cells contains**U**. Then**V**can be eliminated from these two cells. A rectangle that meets this test is also called a Type 4 Unique Rectangle.

An example below:

In this example, observe that the common candidates are 2 and 7, and the extra candidates lie in the ceiling cells **r9c5** and **r9c8**. However, in row 9, the digit 7 must be placed in either **r9c5** or **r9c8**. Since exactly two digits must be placed in **r9c5** and **r9c8**, one being the digit 7 and the other coming from one of the extra candidates, there is no room for these two cells to contain the digit 2. Thus, 2 can be eliminated from both **r9c5** and **r9c8**.

Possible Eureka notation:

(27)LockedSet:r7c58 -UR- (27)r9c58 = (2)r9c237 => r9c58 <> 2

Uniqueness Test 5 is very similar to Uniqueness Test 2, and can be stated as follows:

- Suppose exactly two cells in the rectangle have exactly one extra candidate
**X**, and both cells are located diagonally across each other in the rectangle. Then**X**can be eliminated from the cells seen by both of these cells. This would be called a Type 5 Unique Rectangle.

Note that in this case the rectangle does not have a floor or ceiling.

The raw form of this technique appears to be extremely rare. An example:

Some players will also call the following variant as Uniqueness Test 5:

- Suppose exactly three cells in the rectangle have exactly one extra candidate
**X**. Then**X**can be eliminated from the cells seen by all three cells.

An example below, from the Players' forum:

Note the unique rectangle where rows 2 and 6, and columns 8 and 9 intersect. The common candidates are 5 and 9, and three of these cells, **r2c8**, **r2c9** and **r6c8**, have an extra candidate 6. To avoid the deadly pattern, at least one of **r2c8**, **r2c9** and **r6c8** must contain a 6. As **r1c8** sees all of these three cells, we can eliminate 6 from **r1c8**.

Possible Eureka notation:

(6)r2c89 =[(69)UR:r26c89]= (6)r26c8 => r1c8 <> 6

or:

(6=59)ALS:r26c9 -[UR]- (59=6)ALS:r26c8 => r1c8 <> 6

Uniqueness Test 6 is very similar to Uniqueness Test 4, and can be stated as follows:

- Suppose exactly two cells in the rectangle contain extra candidates, and they are located diagonally across each other in the rectangle. Suppose the common candidates are
**U**and**V**, and none of the other cells in the two rows and two columns containing the rectangle contain**U**. Then**U**can be eliminated from these two cells. This is also called a Type 6 Unique Rectangle.

Note that in this case the rectangle does not have a floor or ceiling.

An example below, from the Players' forum:

An unique rectangle on the digits 6 and 9 plus two other candidates can be found in the intersections of rows 7 and 9, and the columns 3 and 5. In these rows and columns, the candidates for digit 9 can only be found in **r7c3**, **r7c5**, **r9c5** and **r9c3**. Assigning 9 to either **r7c5** and **r9c3** forces 6 in the other corner cells to give one of the two forbidden patterns. So the digit 9 can be safely eliminated from these two cells.

Possible Eureka Notation:

(9)r7c3 = (9-7)r7c5 =[(69)UR:r79c35]= (2)r9c3 => r9c3 <> 9

This uses the strong inference between the two disrupting digits to achieve the same effect.

Suppose we have a rectangle with cells **A**, **B**, **C**, **D**, such that **AD** and **BC** are in opposite corners, and either **AB** or **BD** share a box, and the cells have candidates as follows: **A** has **ab**, **B** has **abx**, **C** has **aby** and **D** has **abz**, where **x**, **y** are one or more candidates, **z** is zero or more candidates, and the same candidate may occur in more than one of these sets. Suppose further that **B** and **D** share a house such that they are the only cells where candidate **a** can occur, and **C** and **D** similarly share a house where only they have **a** as a candidate. Then we can eliminate **b** from cell **D**.

How this works: Suppose cell **D** has the value **b**. Then both **B** and **C** must have the value **a**, since these cells share a house with **D** where they are the only other place an **a** can go. This means that cell **A** must have the value **b**. But since none of the four cells in the rectangle is a given, this is precisely the deadly pattern that allows for two solutions to the Sudoku--we could swap **a** for **b** in the rectangle and still have a valid solution. Therefore, **b** can be eliminated as a candidate for **D**.

Here is an example:

.------------------.------------------.------------------. | 135 19 6 | 24 27 247 | 8 1359 39 | | 358 29 23 | 8 6 1 | 379 34579 3479 | | 17 8 4 | 5 9 3 | 2 17 6 | :------------------+------------------+------------------: | 2 3 18 | 149 17 478 | 479 6 5 | | 4 6 58 | 29 35 278 | 379 379 1 | | 9 7 15 | 146 35 46 | 34 8 2 | :------------------+------------------+------------------: | 18 12 9 | 3 4 5 | 6 27 78 | | 6 5 23 | 7 8 9 | 1 234 34 | | 38 4 7 | 126 12 26 | 5 39 389 | '------------------'------------------'------------------'

The Hidden Unique Rectangle is **A**=**r8c9**, **B**=**r8c8**, **C**=**r2c9**, **D**=**r2c8**, **a**=**4**, **b**=**3**. **r2c8** and **r2c9** are the only cells in **r2** with candidate **4**; similarly, **r2c8** and **r8c8** are the only cells in **c8** with candidate **4**. We therefore can remove candidate **3** from **r2c8**.

Another example from the Programmer's Forum:

The Hidden Unique Rectangle is **r1c26,r2c26**, candidate **4** occurs only once in row 2 and column 6 so candidate **8** can be deleted from **r2c6**.

And one where **z** is zero candidates:

Here the Hidden Unique Rectangle is **r5c89,r8c89**, candidate **4** occurs only once in row 8 and column 9 so candidate **6** can be deleted from **r8c9**.

A Unique Rectangle with extra candidates means that at least one of the extra candidates must be placed in its cell. This means that each of the extra candidates can be the starting point of a Forcing Chain.

In this example, since either **r9c1** contains 3 or **r8c9** contains 2, we can form two Forcing Chains:

r9c1=3 => r4c1<>3 => r4c1=5 => r6c2<>5 => r6c2=7 => r5c2<>7 r8c9=2 => r5c9<>2 => r5c9=7 => r5c2<>7

Both chains result in **r5c2<>7**, so we can eliminate 7 from **r5c2**.

When a Unique Rectangle has exactly two extra candidates, then the elimination of any one candidate means that the other candidate must be placed. This implies that these two candidates have a strong link. Note that this strong link does not necessarily encode a weak link, but the strong-only link may be sufficient to make an inference that causes eliminations in other cells.

This is the same example as above, but recast to use a loop to make the same elimination. First, observe that we have a strong link between 3 at **r9c1** and 2 at **r8c9**. This enables us to construct the following Alternating Inference Chain (in Eureka notation):

(7)r5c2-(7=5)r6c2-(5=3)r4c1-(3)r9c1=(2)r8c9-(2=7)r5c9-(7)r5c2 => r5c2<>7

This also results in 7 being eliminated from **r5c2**.

Due to the similarity of Uniqueness Test 2, Uniqueness Test 5 and its variant, programmers of Sudoku solvers often code them as one single rule or procedure:

- Suppose exactly two or three cells in the rectangle have exactly one extra candidate
**X**. Then**X**can be eliminated from the cells seen by all of these cells.

Uniqueness tests can be adapted to Killer Sudoku with a bit of modification on finding Unique Rectangles. For Killers, the 4 cells in the corners of the rectangle belong to exactly 2 rows, 2 columns, 2 boxes *and 2 cages*.

- Unique Rectangle
- Avoidable Rectangle
- Bivalue Universal Grave
- BUG Lite
- Deadly pattern
- Uniqueness Controversy