### DS: Disjoint Subsets

Look in a row, column or box if there are x number of cells containing x possible numbers, where all the cells has the same possible numbers. Meaning, if a cell has only 2 and 3 as possible numbers, look for another cell in the same row, column or box which also has only 2 and 3 as possible numbers. If this exists, these cells must contain the number 2 or 3 as solution and we can remove 2 and 3 from the row, column or box.

 1 9 2 7 8 6 6 7 9 4 1 2 8 4 7 9 9 5 2 6 7 3 9 1 1 9 6 4 9 1 2 8 4 1 5 9 6 6 2 8 9 1 5 3 7 4

Here you see that R3 has two cells which have 3 and 5 as possible solutions. Therefore we can remove 3 and 5 from the other cells in R3, in this case 3 and 5 from R3C4 and 3 from R3C5.

 1 9 2 345 34 7 8 6 35 35 6 7 358 38 9 4 1 2 8 35 4 12356 236 126 7 35 9 347 378 9 1468 5 1468 2 38 37 245 58 6 7 248 3 9 58 1 2357 1 35 28 9 28 6 4 357 9 357 35 346 3467 46 1 2 8 37 4 1 238 2378 28 5 9 6 6 2 8 9 1 5 3 7 4

This doesn't work only for 2 numbers in 2 cells. You can do it for the same numbers in 3 cells, 4 cells etc. This sudoku can be solved using DS and SiSo.