IBM Ponder This #191 · March 2014

N lamps are set in a circle, and for each integer M you have a tool that can toggle the state (on/off) of any set of M consecutive lamps.

Find a possible N which satisfies the following statements:

• The sum of its digits is less than 10.
• By applying the tool for M=105 several times, we can toggle a single lamp.
• If we remove one lamp and start from a random initial setting for the remaining N-1 lamps, the probability that there exists a way to apply the tool for M=32 several times and switch all the lamps off is positive and less than 0.0011%.

Solution

Best opened after a real attempt

To be added.