# #7: The Visible Grid Point Problem

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?

# #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:

Examples:

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

Non-examples:

• 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.

# Analytic Number Theory and Other Updates

Over the winter break I self-studied Tom Apostol’s Introduction to Analytic Number Theory book and completed the first three chapters, as well as the majority of the fourth. I posted the solutions, in pdf format this time, to the Solutions page. I chose this subject because Cornell currently does not have any course on analytic number theory, though it does have a graduate course in algebraic number theory, which I am taking this semester. More of that information is in a post on my main blog.

As stated in that post, I am now a double major in mathematics and computer science. This semester I am also planning to take all my notes in real-time LaTeX, aka LiveTeXing, as someone pointed out. The notes I have uploaded to Scribd, as it is more organized there and easier to view at a glance. They should be uploaded by the end of the same day of each lecture. I may upload them directly to this blog as well for centralization.

Courses that I am LiveTeXing: CS 4820, CS 4850, Math 4340, Math 7370.

I am also taking CS 3410, but the professor is posting all the powerpoints online, so it would be redundant to LiveTeX it.