Thirteen pirates go
on an extended voyage, pillaging and plundering from Africa to Asia.
By the end they have quite a stash--too much to take back with them.
They decide to lock it in a chest, leave the chest on an island, and
come back for it a year later. Of course, not being terribly
trusting, they want to ensure that none of them can come back early
and claim the treasure for himself. They could just put 13 locks on
it and each take a key, but a pirate's life is dangerous--they may
not all be around in a year. What they want is for any majority of
the original thirteen to be able to open the chest, while any fewer
will be locked out. How many locks will it take for them to achieve
this? The locks they are using are quite simple. Each key opens
only one lock (no master keys), but keys can be duplicated, so
multiple pirates can have keys to the same lock.

Prepared Prisoners

There are 100 prisoners in solitary confinement in a jail. The
head guard decides to play a game with them. He will, at random
intervals, randomly select one of them and take him out of his cell
to a room with two lightbulbs. The prisoner can then choose to
toggle the state of the lightbulbs (or he can do nothing). Before
returning to his cell, he may choose to declare that all prisoners
have been to the room. If he is correct, all the prisoners will be
freed. If he is wrong, they will be executed. The prisoners cannot
see the lightbulbs from their cells and they do not know the state
of the lightbulbs at the start of the game. But they are allowed
one evening together to decide on a strategy. What strategy should
they follow to ensure that they all get released?

Directed Graphs

Prove that if each node in a directed graph has indegree > 0, then
the graph must contain a cycle.

Restricted Rectangles

You have a rectangle, which is composed of smaller rectangles.
Each of the interior rectangles has the property that at least one of
its sides has an integer length. Prove that the larger rectangle has
the same property.

Commensurate Cake

You are given a (two-dimensional) rectangular cake. Someone
takes a rectangular piece from this cake. The piece can have any
dimensions and orientation as long as it is rectangular. Your job is
to cut the remaining cake into two pieces of equal area. You are only
allowed to make one cut. And all you have to help you decide where to
cut is a straightedge.

Hinged Mirrors

Consider two infinite planar mirrors as shown below. Prove that
regardless of the angle between the mirrors, any beam of light that is
directed at the mirrors bounces off of them only a finite number of
times. That is, eventually the ray of light is such that it never
again intersects a mirror. The ray of light is assumed to come from a
"perfect laser", so it is infinitely thin. Also, the ray is never
directed straight at the vertex, as the reflection from that point is
undefined.

Coloring the Plane

Let each point in the Euclidian plane be colored either red or
blue. Prove that there are two points of the same color which are
unit distance apart.

Coloring the Plane (arbitrary distances)

Show that for any 2-coloring of the plane (as per the previous
problem), there is a color for which you can find two same-colored
points distance d apart for ALL distances d.

Coloring the Plane (3 colors)

Show that for any 3-coloring of the plane you can find two points
of the same color which are unit distance apart. (Same as the first
coloring problem, but with 3 colors).

Covering a Rectangle

Given a covering of a rectangle using n circles of radius r,
prove that the same rectangle can be covered using 4n circles of
radius r/2.

Sum and Product

Two integers, m and n, each between 2 and 100 inclusive, have been
chosen. The product, mn, is given to mathematician P. The sum, m + n,
is given to mathematician S. Their conversation is as follows:

S:

"I know that you don't know the sum."

P:

"Now I know the sum."

S:

"And now I know your product."

What were the numbers?

Lightswitches

There is a line of 100 lightswitches, all initially off. If you toggle
every lightswitch, then toggle every other lightswitch, then every third
lightswitch, and so on until the only switch you flip is the last one, which
switches will be on?

Probability Conversion

Given a biased coin which lands heads with probability 1/e, construct an
experiment which, should it terminate, succeeds with probability 1/pi. The
experiment can take an unbounded amount of time.