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Solving Techniques 8
Simple Chain

Before considering simple chains, it’s important to understand strong links and weak links.

Strong link

As shown below, a link where only two numbers are possible in a row, column or box, is called a strong link. In the diagram, it is between [1] and [2].

Weak link

As shown below, a weak link is a link where there are more than three numbers that are possible within a row, column or box. [1] and [2] are in three possible cells, which creates a weak link.

Simple Chain

A simple chain is when you focus on a number and look for a
cell - strong - cell - weak - cell - strong - cell
or
cell - strong - cell - weak - cell - strong - cell - weak - cell - strong - cell
pattern that begins and ends in a strong link. In the intersecting point between the beginning and end of the link, that number can’t be entered (it also works when weak links are strong in the pattern).

As an example, it will end up looking like the diagram above.

If a [6] goes in the first cell, [R6C4], it obviously can’t be in [R6C2].

If [7] goes in the first cell, [R6C4], then [6] will go in the last cell, [R9C2], so again, [6] can’t be entered in [R6C2].

The [1]-strong-[not 1]…. pattern will end up in [not 1]-strong-[1]-weak-[not 1]-strong-[1], and so a [1] will always come in the beginning or end of the chain.

This is the technique, but it is quite difficult to find the pattern, and unless you are really looking for it, it will be hard to find.

Names of cells in Sudoku

R1C1	R1C2	R1C3	R1C4	R1C5	R1C6	R1C7	R1C8	R1C9
R2C1	R2C2	R2C3	R2C4	R2C5	R2C6	R2C7	R2C8	R2C9
R3C1	R3C2	R3C3	R3C4	R3C5	R3C6	R3C7	R3C8	R3C9
R4C1	R4C2	R4C3	R4C4	R4C5	R4C6	R4C7	R4C8	R4C9
R5C1	R5C2	R5C3	R5C4	R5C5	R5C6	R5C7	R5C8	R5C9
R6C1	R6C2	R6C3	R6C4	R6C5	R6C6	R6C7	R6C8	R6C9
R7C1	R7C2	R7C3	R7C4	R7C5	R7C6	R7C7	R7C8	R7C9
R8C1	R8C2	R8C3	R8C4	R8C5	R8C6	R8C7	R8C8	R8C9
R9C1	R9C2	R9C3	R9C4	R9C5	R9C6	R9C7	R9C8	R9C9

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