No two neighboring bulbs lit

Each of the 99 bulbs in a 3×33\times 3 grid is independently on with probability 12\tfrac12. What is the probability that no two edge-adjacent bulbs (cells sharing a side) are both on?

Show hints (2)+
  1. All 29=5122^9=512 patterns are equally likely; count the valid ones (independent sets of the grid).
  2. A row-by-row transfer count gives 6363 independent sets, so 63512\tfrac{63}{512}.

Answer

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0.123 (± 0.002)

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Asked at: Jane Street, Two Sigma

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