- Code: Select all
`5 . | . . | 1 . | 8 .`

. . | . . | . . | . .

6 . | 7 . | . . | . 2

. . | 3 . | . 4 | . .

-----------------------

. . | 8 . | . 5 | . .

7 . | . . | . 6 | . 4

. . | . . | . . | . .

. 2 | . 1 | . . | . 3

... and was very pleased with the stage that immediately follows at this point:

- Code: Select all
`5 . | . . | 1 . | 8 .`

. . | 1 . | . . | . .

6 . | 7 . | . . | . 2

. . | 3 . | . 4 | . .

-----------------------

. . | 8 . | . 5 | . .

7 . | . . | . 6 | . 4

. . | . . | . 1 | . 8

. 2 | . 1 | . . | . 3

Here I relabel four of the blank cells A, B, C and D:

- Code: Select all
`5 . | . . | 1 . | 8 .`

. . | 1 . | . . | A .

6 . | 7 . | . . | B 2

. . | 3 . | . 4 | C D

-----------------------

. . | 8 . | . 5 | . .

7 . | . . | . 6 | . 4

. . | . . | . 1 | . 8

. 2 | . 1 | . . | . 3

Now clearly either B, C or D is the position for 1 in the top-right box. But observe that A and B are 3 and 4 in some order. So either C or D is 1. Suppose C is one; one can see almost immediately that this would prevent a 1 being entered in the bottom-left box. It follows that D must be 1. The puzzle progresses from there to a unique solution.

What is the name of the technique I used? I was very pleased to find it.