Some of you who read my rambling blog might occasionally wonder what it is that I actually do as a math grad student. I’ve decided that it’s high time I tried to give a reasonably coherent explanation, so here goes.
Most of math, or at least the math I’m interested in, spends a lot of time redefining what it means for two things (numbers, groups, rings, spaces, and other technical math constructions) to be the same. That’s right–I spend all day deciding what “=” means, depending on the context.
Question 1: What does 2*2-2 = 4+6-8 mean to you?
For example, suppose I’m interested in numbers that have a remainder of 1 when you divide them by 3. If that’s the only property I really care about, then I might say something silly like [1] = [4], where those brackets remind me that I’m working with a special equivalence class, i.e., all the numbers that have remainder 1 when divided by 3.
Question 2: Can you find all the equivalence classes with respect to division by 3? (Hint: take a look at the numbers on a touch-tone phone.)
Other times, I’m looking at a space. Is a donut sitting flat on a table the same as a donut balanced on its edge? To me they are. Is a donut’s surface the same as a coffee cup’s surface? Not usually, but sometimes all I care about mathematically is the fact that there’s a big hole in the space I’m looking at. Geometrically, of course, they’re very different–a coffee cup has a large, well, cup in it, and a donut certainly doesn’t. So from the perspective of one branch of math (algebraic topology), “donut = coffee cup,” while in other branches (geometry, calculus), “donut ≠ coffee cup.”
Question 3: Can you think of two objects that might be the same with respect to certain conditions, but different with respect to others?
So that’s part 1 of what I do all day. It’s a small part, but it’s the first question a mathematician asks when thinking about some type of mathematical object: “How do I define what X = Y really means?”
Part 2 of what I do will discuss different ways to put together things that mathematicians know to get new information–that’s right, we’re talking about proofs! As a primer, I suggest you go play some sudoku, because I’m probably going to make a lot of analogies to a sudoku game.
–Answers–
Answer 1: That’s up to you!
Answer 2: The equivalence classes are [0] (everything that 3 divides evenly, i.e., with no remainder), [1], and [2] (everything that 3 divides with a remainder of 2). Formally, we can define these as
[0] = {0,3,6,…} = {3n : n is an integer}
[1] = {1,4,7,…} = {3n+1 : n is an integer}
[2] = {2,5,8,…} = {3n+2 : n is an integer}.
Answer 3: The first thing that came to my mind was “Apples and Oranges!” They’re the same in that they’re both fruit, of course, but they’re also the same in that they are “homeomorphic” to a solid ball–that means I could smush them around to look like a 3D ball. They’re different in the sense that an apple has “saddle points” while an orange (ideally) is completely spherical. And apples taste better.