The names of 100 prisoners are placed in 100 wooden boxes, one name to a box, and the boxes are
lined up on a table in a room. One by one, the prisoners are led into the room; each may look
in at most 50 boxes, but must leave the room exactly as he found it and is permitted no further
communication with the others.
The prisoners have a chance to plot their strategy in advance, and they are going to need it,
because unless every single prisoner finds his own name all will subsequently be executed.
Find a strategy for them which has probability of success exceeding 30%.
Comment: If each prisoner examines a random set of 50 boxes, their probability of survival
is an unenviable 1/2^{100} ∼ 0.0000000000000000000000000000008. They could do worse—if they all
look in the same 50 boxes, their chances drop to zero. 30% seems ridiculously out of reach—but
yes, you heard the problem correctly!
http://puzzling.stackexchange.com/questions/16/100prisonersnamesinboxes
Check especially Anachor's explanation: ". . . we just need to find the probability that there is a cycle of 51 or longer.
. . . the probability that there is a cycle of length 51 or longer is just
1/51+1/52+1/53+...+1/100≈0.688172
, so the probability of the opposite is 0.3118280.311828 which is above 30%."

Posted by xdog
on 20170412 12:42:48 