UPDATE (6 January): Clarification: Answer either
of the riddle's two parts for credit. The idea is that people with no prior
familiarity with Part 1 can get credit by solving only it, whereas people with
prior familiarity with Part 1 can get credit by solving Part 2.
UPDATE (2 January): A few readers expressed prior familiarity with this generalization of the classic genie riddle (I'll give references on the solution page), so I decided to add another question to it. See Part 2, below.
This month's riddle is a generalization, due to Oded Margalit (Thanks, Oded!), to a classic puzzle.
You've been trapped by a genie in a dark room, and your only hope of escaping is to solve the genie's puzzle. The genie places n cups equally-spaced along the perimeter of a circular table in front of you. You can feel the cups, but because the room is dark you cannot tell if the cups are right-side-up or upside-down.
The genie allows you to turn over any subset of the cups that you want (potentially turning them right-side-up, if their original orientation was upside-down), and, after doing so, to let go of the cups and ask the genie to check their orientation. If they are all right-side-up, the genie will let you go. If not, you can try again. The genie allows you to try as many times as you want.
The catch, however, is that as soon as you let go of the cups and ask the genie to check them, the genie swivels the table around, so the next time you touch the cups, they will have rotated an unknown number of degrees.
To make matters worse, the genie -- being a genie -- knows exactly what strategy you are planning, and will rotate the table in exactly the way that will prolong the game as much as possible.
This month, for credit, answer all of the following questions (in either or in both of the following two parts):
Part 2:The genie's actions, as described above, permute the cups by taking each cup that was at position x and switching it to position (x+k) mod n, where the genie chooses a new k after every check.
Another way to describe the same process is to say that the genie has, at its disposal, a set of permutations
defined by πk(x)=(x+k) mod n, and the genie merely picks, after each check, one of these to apply.
The question: what is the richest set of permutations that the genie can use without this affecting the success of the strategy you discovered in Part 1? Please supply the following.
List of solvers:
Part 1:Guangda Huzhang (6 January 20:22)
T.T. Msh (16 January 22:22)
Harald Bögeholz (17 January 08:36)
Part 2:T.T. Msh (17 January 17:17)
Elegant and original solutions can be submitted to the puzzlemaster at riddlesbrand.scso.com. Names of solvers will be posted on this page. Notify if you don't want your name to be mentioned.
The solution will be published at the end of the month.
Back to main page