There are various variants with less than 81 fields. The same
rules apply as with the "normal" Sudoku. Instead of
Unterquadrate there are subregions, in which the numbers may
occur only once.
3x3
Sudoku
9 fields into which one the numbers from 1 to 3 must be filled.
4x4
Sudoku
16 fields into which one the numbers from 1 to 4 fill must.
5x5
Sudoku
25 fields into which one the numbers from 1 to 5 fill must.