Locate some "light bulbs" in the grid such that every white cell is "lit up". Each bulb occupies a single white cell, and lights up its own cell, as well as white cells in the four orthogonal directions until the light beam encounters a black square or the edge of the grid. A bulb may not illuminate another light bulb. All white cells must be lit up by at least one bulb. A given number in a black cell indicates how many cells orthogonally adjacent to it are occupied by bulbs.
(Rules and example from PGP IB, rays from the bulbs are only here for illustrative purpose)
History of the puzzle
First appeared on Nikoli volume 95 (2001). Invented by あさおきたん ("Asaokitan"). Original name 美術館 ("Bijutsukan") means "art museum" and it is possibly a reference to art gallery problem, a well-known problem in computational geometry.
The name Akari means "light" in Japanese. It is an English name given by Nikoli Co., Ltd. to 美術館.
Invented by Naoki Inaba (Japan) in 2011 under the name アマテラス ("Amaterasu": the Sun goddess of Japanese myths). The name Regional Akari was given by Bram de Laat (Netherlands) in 2012.
Locate some light bulbs in the grid such that every white cell is "lit up". A bulb may not illuminate another light bulb. All white cells must be lit up by at least one bulb. Each region contains exactly one lightbulb.
(Example from WPC 2018 IB)
First appeared on Tawan Sunathvanichkul's (Thailand) blog post in 2014. This puzzle is a modification on an earlier puzzle by テラシティ蔵前 (Terra-city Kuramae) on Nikoli vol 135 (2011).
Place coloured (either green, blue or red) light bulbs into white cells so that all white cells are illuminated. Each bulb illuminates all white cells that are in the same row and column as itself, until it reaches a wall or a black cell. No two bulbs can illuminate each other.
Shaded cells with a coloured circle represent the additive mixture (pink = red + blue, yellow = red + green, cyan = blue + green) of colours of the lights illuminating (not restricted to adjacent ones) that square.
(Rules and example from WPC 2015 IB)
Appearances in the past WPCs
- WPC 2019/World Cup Round 3 by Silke Berendes
- WPC 2018/Round 2 by Jan Zvěřina
- WPC 2018/Round 11 (Regional Akari) by Jan Zvěřina
- WPC 2017/Round 3 by Rohan Rao
- WPC 2017/Round 7 by Prasanna Seshadri
- WPC 2017/Round 10 ("Optimizer") by Deb Mohanty
- WPC 2017/Round 13 (Regional Akari) by Rajesh Kumar
- WPC 2016/Round 16 by Matúš Demiger
- WPC 2015/Round 5 (Color Akari) by Tawan Sunathvanichkul
- WPC 2014/Round 1