Immeasurable Sets in R

The following example was brought up in two different classes that I am taking, within a couple days of each other. It is the classic example of an immeasurable set on the interval [0,1]. Now in math, words like measure and measurable have technical definitions, and lead to bizarre results like the Banach-Tarski paradox. I’m going to give a fairly informal explanation here. Continue reading

#7: The Visible Grid Point Problem

A link to this article in pdf format: [pdf]

Here is a difficult probability question:

Suppose you are standing on an infinitely large square grid at the point (0,0), and suppose that you can see infinitely far but cannot see through grid points. Given a random grid point z = (x,y), where x and y are integers, what is the chance you can see z?

Continue reading

On the Connectivity of the Reddit Community

Here is the link to the paper [pdf].

Abstract: The main statistical result of this paper is that of the reddit users who have upvoted at least one post, 31.4% of them have upvoted only one post and, moreover, have been the only person to upvote it. Conversely, 22.6% of posts which have been voted on at all have received only downvotes and no upvotes. In addition, 67.4% of users are connected in a giant component of upvoting common posts, while 91.4% of posts are connected by having been upvoted by common users.

#6: The Sum of Two Squares

In the Algebraic Number Theory course that I am taking, we covered the problem of when a number is the sum of two squares. I wrote a short expository paper [pdf] to go with it. Here are the positive integers up to 15 which can and cannot be written as the sum of two squares:


  • 1 = 1² + 0²
  • 2 = 1² + 1²
  • 4 = 2² + 0²
  • 5 = 2² + 1²
  • 8 = 2² + 2²
  • 9 = 3² + 0²
  • 10 = 3² + 1²
  • 13 = 3² + 2²


  • 3
  • 6
  • 7
  • 11
  • 12
  • 14
  • 15

Can you find the pattern? (Note: This is a very hard question. You may need to write down more examples to start noticing it.)

I have also posted some hints.