Akari,
place bulbs, light the board.

Akari — also called Light Up or Bijutsukan — is a single-player logic puzzle by Nikoli, the same publisher that gave the world Sudoku and Slitherlink. You place light bulbs on the white cells of a grid so that every white cell is lit, no two bulbs see each other along an open row or column, and every numbered black wall has exactly that many bulbs sitting in its orthogonal neighbours.

Tap a white cell to cycle it through three states: empty, lit bulb, then a small dot. The dot is your own marker for "no bulb here" — useful for tracking deductions when you have ruled a cell out. Black cells are walls and cannot be clicked; numbered walls show how many of their four neighbours must contain a bulb.

A bulb illuminates its own cell and every white cell in the same row and column, stopping when it hits a black wall or the board edge. A white cell counts as lit if at least one bulb shines on it. Two bulbs that share the same unbroken row or column segment would see each other — that is forbidden.

A number from 0 to 4 on a black wall is the exact count of bulbs that must sit in the four cells immediately above, below, left, and right of it (those that exist within the board). A 0 means none of those neighbours may hold a bulb. Walls without a number have no constraint of their own — they still block light, they just do not demand any particular neighbour count.

Easy is a 7×7 grid — a quick five-minute solve once you get the rhythm. Medium is 9×9 and rewards deeper deduction; Hard is 11×11 with sparser hints and longer chains of inferences. The clue density is fixed across all tiers; size alone scales the complexity.

Look at the extreme clues first. A 4-wall in the open immediately fills its four neighbours with bulbs. A 0-wall lets you mark every neighbour as "no bulb here". For 3-walls in a corner, the three neighbours that exist are all bulbs. After that, scan for cells that have only one possible row/column run that could light them — those rows or columns must contain a bulb somewhere.

Yes. The generator verifies uniqueness by counting solutions and rejects any puzzle whose constraint set permits more than one bulb arrangement. Hints draw from the canonical solution, so leaning on one never desyncs you from a valid completion.

Yes. A fresh puzzle is generated each time you change difficulty or hit "New puzzle". There is no daily limit, no streak counter, and no leaderboard — solve as many as you like.

Yes. Cells are sized for thumb taps, and the single-tap cycle (empty → bulb → dot → empty) keeps interaction simple. The 7×7 easy size is the most comfortable on a phone; medium and hard work best on a tablet or laptop where the grid is roomier.