The New Sudoku Players' Forum

[urhegyi]

This is the extreme sudoku of 26/05/2021 found at http://manifestmaster.com/DocsExtreme/20210526/index.html

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4.......9.1.4...5.3...2.7...5.......2...3.6...9.7.1.4.7.....4.6.....3.9.....4.1..

What do you think of this technique, is it already known?

After singles(R1C4=3;R3C9=4;R4C7=9)and basics I come to this state:

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.--------------------.--------------------.------------------.
| 4     2678  25678  | 3      15678  5678 | 28    16    9    |
| 689   1     26789  | 4      6789   6789 | 238   5     238  |
| 3     68    5689   | 15689  2      5689 | 7     16    4    |
:--------------------+--------------------+------------------:
| 168   5     134678 | 268    68     2468 | 9     378   1378 |
| 2     478   1478   | 589    3      4589 | 6     78    1578 |
| 68    9     368    | 7      568    1    | 2358  4     2358 |
:--------------------+--------------------+------------------:
| 7     238   128    | 12589  1589   2589 | 4     238   6    |
| 1568  2468  12468  | 1268   1678   3    | 58    9     578  |
| 5689  2368  2689   | 268    4      2678 | 1     2378  3578 |
'--------------------'--------------------'------------------'

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R4C1 has two possible states 1 or not 1. if 1 then R4C9<>1 and Triple 378 and Pair 25 in box 6 leading to R6C5<>5 and Pair 68 in column 5 and box 5 that leads to R4C4=2 and R4C6=4. if not 1 then pair 68 in row 4 and box 5 also leading to R4C4=2 and R4C6=4.

[denis_berthier]

In Sudoku Explainer parlance, it's some kind of forcing chain. But, AFAIK, SE would consider the 3 values of r4c1 separately.

In any case, there is a much simpler next step:
z-chain[4]: c1n9{r2 r9} - c1n5{r9 r8} - r8c7{n5 n8} - b3n8{r1c7 .} ==> r2c1 ≠ 8

[yzfwsf]

ALS-XY-Wing can obtain the same deletion effect.
Almost Locked Set XY-Wing: A=r4c15{168}, B=r4c13589{134678}, C=r5c238{1478}, X,Y=1, 4, Z=68 => r4c46<>6 r4c46<>8