Pigeonhole Principle/Solutions
These are the solutions to the problems related to the pigeonhole principle.
Contents
Introductory
If a Martian has an infinite number of red, blue, yellow, and black socks in a drawer, how many socks must the Martian pull out of the drawer to guarantee he has a pair?
The Martian must pull 5 socks out of the drawer to guarantee he has a pair.
In this case the pigeons are the socks he pulls out and the holes are the colors of socks.
Thus, if he pulls out 5 socks, the Pigeonhole Principle states that some two of them have the same color.
Also, note that it is possible to pull out 4 socks without obtaining a pair.
Intermediate
M1
The maximum number of friends one person in the group can have is 4, and the minimum is 0. If all of the members have at least one friend, then each individual can have somewhere between 
to 
friends; as there are 
individuals, by pigeonhole there must be at least two with the same number of friends. If one individual has no friends, then we can repeat the above argument for the remaining 4 individuals, and if more than one individual has no friends then we are done.
M2
For the difference to be a multiple of 5, the two integers must have the same remainder when divided by 5. Since there are 5 possible remainders (0-4), by the pigeonhole principle, at least two of the integers must share the same remainder. Thus, the answer is 1 (E).
Olympiad
O1
By the Triangle Inequality Theorem, a side of a triangle can be less than the sum of the other two sides. Assume that the first two segments are as short as possible (1 inch). For a triangle to not exist, the next term must be 2, then 3, 5, 8, and 13 (this is the Fibonacci sequence). The 7th term is 13, which is greater than 10 inches.
O2
For the difference to be a multiple of 7, the integers must have equal modulo 7 residues. To avoid having 15 with the same residue, 14 numbers with different modulo 7 residues can be picked (
). Thus, two numbers are left over and have to share a modulo 7 residue with the other numbers under the pigeonhole principle.
O3
See this article.
O4
Inscribe a regular 
-gon, it will divide the circle into equal arcs. The length of the side of this -gon is 
, and this is an upper bound on the distance of any two points on the arc. From the pigeonhole principle one of the arcs contains at least two of the points.
O5
The pigeonhole principle is used in these solutions (PDF).
O6
This problem needs a solution. If you have a solution for it, please help us out by adding it.
O7
This problem needs a solution. If you have a solution for it, please help us out by adding it.
