The Pigeonhole Principle

The pigeonhole principle states that if a group of pigeons flies into a set of pigeonholes, and there are more pigeons than pigeonholes, then there must be at least one pigeonhole with two pigeons in it. More generally, if k + 1 or more objects are placed into k boxes, then there is at least one box containing two or more of the objects. Despite its seeming simplicity (perhaps obviousness), it can be used to solve a surprising range of problems in probability, number theory, and computer science, just to name a few. See if you can use it to solve the following three problems.

1. (Warm up) A drawer contains a dozen blue socks and a dozen black socks, all unmatched. If the room is dark, how many socks do you have to take out to be sure you have a matching pair?
2. Prove that there are at least two people in Tokyo with exactly the same number of hairs on their heads.
3. Prove that if five distinct integers are selected from the numbers 1 through 8, there must be at least one pair with a sum equal to 9.

Click below to see the answers.

1. (Warm up) If you've only heard one problem involving the pigeonhole principle, it was probably the classic sock drawer problem. You only need to pick three socks to make sure you have one matching pair. When you pick two socks, you might already have a matching pair, or you might have one of each color sock. Selecting one more sock ensures that you have at least two socks of one color or the other.
2. The Tokyo hairs problem sounds like something you might be asked as a "brain teaser" interview question. If you're stuck in an interview, then the first step is to show off your estimating skills. Since we're not in that situation, we can just use Google to find out that there are about 100,000 hairs on the average human head, and that Tokyo is home to about 13.6 million people. That's more than enough people for our proof. For the sake of simplicity, let's say that 200,000 is the maximum number of hairs a person can have on their head. Then, if you select 200,001 people who happen to each have a distinct number of hairs on their heads (zero is a valid number of hairs to have on your head), you only need one more to ensure that two people have the same number of hairs. (Note: This puzzle will work with any city larger than 200,001 residents.)
3. To see how the pigeonhole principle applies to this problem, you just need to group the numbers 1 through 8 in pairs that sum to 9. {1,8}, {2,7}, {3,6}, {4,5}. Now, if I select four distinct numbers from that range, I might select one number from each of the four pairs. If I select a fifth number, then I must complete one of the pairs that sums to 9.