Fill some cells with a number from 1 to 4. All numbered cells must be orthogonally connected. Orthogonally adjacent cells cannot contain the same number. Each number must indicate the number of orthogonally adjacent numbered cells. Some numbers are given to you.
(Rules and example from PGP IB)
History of the puzzle
First appeared on Nikoli vol 40 (1992). Invented by "Tea.M". Originally named 数字増殖コロニー (Suji Zoshoku Koroni, "Numbers Proliferation Colony"), but Nikoli renamed it to 数コロ (Sukoro).
Also called Infection. This alternative name may be a reference to bootstrap percolation, a notion in combinatorics.
Appearances in the past WPCs
- WPC 2016/Round 2 by Matej Uher