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Light Up: A Satisfied Number

A number that already has all its bulbs has no more beside it. A 0 has all its bulbs from the start, which makes it the best first move in most puzzles.

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This is one of the Light Up techniques. Each page shows its technique on the same 12×12 from the game, Hard level 4.

The rule

  1. Find a number, and count the bulbs touching it side by side.
  2. If that’s the number, every other white square touching it gets a dot.

In a real puzzle

On the 12×12, it’s the very first move. The 0 in row 2, column 9 has four white squares beside it, and none of them can have a bulb.

Hard level 4, step 1 number-satisfied: no bulb on 4 (from row 2, column 9) 2 1 0 1 2 1 0 1 2 0 1 1 1 1 4 0 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12
Step 1: the 0 rules out the four squares touching it.

How often you’ll need it

On 46 of the 50 Easy levels, 47 Medium and all 50 Hard: 2.5 times per Easy level on average, 4.8 on Medium and 9.3 on Hard. A 0 is the first move on 133 of the game’s 150 levels. It’s 7 of the 42 steps on the 12×12 here.

Next steps

Questions

Does a 0 stop light?

Yes, like every black square. The 0 only adds that none of the four squares touching it can have a bulb.