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infinite sequence of numbered lights (2023 Auckland MO p6)

Source:

March 26, 2024
combinatoricsnumber theory

Problem Statement

Suppose there is an infi nite sequence of lights numbered 1,2,3,...,1, 2, 3,..., and you know the following two rules about how the lights work: \bullet If the light numbered kk is on, the lights numbered 2k2k and 2k+12k + 1 are also guaranteed to be on. \bullet If the light numbered kk is off, then the lights numbered 4k+14k + 1 and 4k+34k + 3 are also guaranteed to be off. Suppose you notice that light number 20232023 is on. Identify all the lights that are guaranteed to be on?