Place a number into each cell so that each bold region contains the numbers from 1 to n, where n is the number of cells in the region. Cells containing the same number x within the same row (or column) must have at least x cells between them in that row (or column). (For example, cells containing "1" cannot touch along an edge, cells containing "2" cannot touch or have exactly one cell between them in the same row or column, and so on.) Some numbers may be already filled in the grid.
(Rules and example from PGP IB)
History of the puzzle
First appeared on Nikoli volume 73 (1998). Rules suggested by ゲサク ("Gesaku"). Originally entitled 波及効果 ("Hakyuu Koka," namely "Ripple Effect").