Sudoku Academy

Before we can move on to more advanced Sudoku solving techniques, you need to learn about Strong and Weak links.

Both Strong and Weak Link are between 2 candidates (or pencil marks).

All cell candidates has multiple Links with other cell candidates - some will be Strong, some will be Weak.

Links by themselves aren't useful, but when you combine multiple Links together, they allow us to remove candidates and move forward with harder Sudoku puzzles.

If making one candidate false means that another candidate must be true, we say that there is a Strong Link between these candidates.

Looking at row 5 in this Sudoku grid, we can see that there are two '4' candidates in this row. If we decide that the '4' candidate on the left is false (i.e. that cell can't contain a '4'), then that automatically means the '4' candidate on the right must be true (Single Position).

Likewise, if we decide that the '4' candidate on the right is false, this automatically means the '4' candidate on the left is true.

We can draw a solid line here to show that there is a Strong Link between these two candidates

This is another example of 2 Strong Links.

In column 4, there is a Strong Link between the '7' candidates. If the top '7' candidate is false, the bottom '7' candidate must be true. If the bottom '7' candidate is false, the top '7' candidate must be true.

In column 8, there is a Strong Link between the '9' candidates. If the top '9' candidate is false, the bottom '9' candidate must be true. If the bottom '9' candidate is false, the top '9' candidate must be true.

Strong Links can also appear between values inside the same region.

The same principle applies, if one candidate being false automatically means another candidate must be true, there is a Strong Link between these two candidates.

Individual Strong Links aren't useful to us, but become very powerful when multiple Links are combined together.

We say that two candidates share a Weak Link if one candidate being true causes the other candidate to be false.

Looking at the third row, if the '8' in column 1 is true, it will cause the '8' in column 6 to be false. (It will also cause the '8' values in columns 4 and 7 to be false, these are also valid Weak Links).

In column 6, the '9' candidate appears 3 times, there are Weak Links between these - this diagram only shows one of them.

In the bottom-middle region, the '1' candidate appears 3 times, there are Weak Links between these.

Weak Links are everywhere.

We can find Strong and Weak Links everywhere in a Sudoku Puzzle.

They are not useful individually, but the more advanced solving techniques use chains of Strong and Weak Links together to make deductions.

The first few techniques only require 2 or 3 Links chained together, but more advanced techniques will require you to put a much higher number of Links together in a chain.