SUDOKU puzzle

Locked Candidates

Different names

The technique of locked candidates was discovered by many sudoku players at the same. This has caused this technique to be known by many different names:

Block-Block Interaction
Row/Column-Block Interaction
Line-Box Interaction
Pointing Pairs

The latter name is not fully accurate. There could be 3 locked candidates in stead of 2.

 

The rule

Sometimes a candidate within a box is restricted to one row or column. Since one of these cells must contain that specific candidate, the candidate can safely be excluded from the remaining cells in that row or column outside of the box.

Sometimes a candidate within a row or column is restricted to one box. Since one of these cells must contain that specific candidate, the candidate can safely be excluded from the remaining cells in the box.

 

Example 1

     
     
     
     
     
     
     
     
     
16 157
8 		157
134 134
578 		2
146 9 145
9 125
78 		125
8
6 145
78 		158
245 124
58 		3
4 3 156
8
9 58 158
268 256
8 		7
     
     
     
     
     
     
     
     
     

Watch digit 2.

 

 

Solution:

Digit 2 is a locked candidate for R6C7 and R6C8. 2 can be excluded as a candidate in R6C4 and R6C5.

 

Example 2

4 15 7
235 8 135
6 125
9 		159
     
     
     
     
     
     
579 145
79 		145
69
27 3 18
259 259 568
9
     
     
     
     
     
     
1 457
9 		345
9
357 6 345
8 45 2
     
     
     
     
     
     

Watch digit 9.

 

 

Solution:

C1 has candidate 9's only in the middle box. Therefore, since one of these cells must be a 9 (otherwise the column would be without a 9), 9's can safely be excluded from all cells in this middle box except those in C1.

 

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Example 3

39 6 39
149 179 8
2 357 345
1 278 4
3 27 5
6 78 9
278 2 789
6 249 79
478 348 1
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     

Watch digit 3.

 

 

Solution:

The number 3 only appears as a candidate in the part of that row that is inside the top left block, cells R1C1 and R1C3 Therefore, we could remove 3 as a possibility from all the other cells in that block. This means that we remove 3 from R3C2 and R3C3.

 

Example 4

3 5 6
7 129 129
8 4 19
     
     
     
     
     
     
24 3 245
6 12 124
7
9 8 124
57
     
     
     
     
     
     
5 6 124
89
14 19 149
124 7 3
     
     
     
     
     
     

Watch digit 1.

 

 

Solution:

Looking at C1, the number 1 only appears as a candidate in the part of that row that is inside the bottom left block, cells R8C1 and R9C1. Therefore, we could remove 1 as a possibility from all the other cells in that block. This means that we remove 1 from R7C3, R8C2 and R8C3.

 

 

 

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