Where to Begin on a 9x9
List the pairs a black dot can join: 1 with 2, 2 with 4, 3 with 6, 4 with 8. A cell between two black dots is often a 2 or a 4, the digits that belong to two doubles. And 5, 7 and 9 never touch a black dot.
Every Pair Is Marked
The puzzle shows all its dots, so a bare border also speaks: the two cells on either side are neither neighbors in counting nor a digit and its double. On grids with very few digits, those bare borders do much of the solving.
Three Levels for the Full Grid
Easy keeps twenty-eight digits and plays like a sudoku with hints. Medium keeps fourteen. Hard removes every digit the dots can replace, which usually leaves one or two on the whole grid.