by Jonathan Kujawa
Functions are machines that take in a number as an input, apply some rule to that input, and generate an output. The input might be cost, temperature, wind speed, a politician’s favorability rating, or whatever you like. The output could tell you the resulting profit, windchill, chance of a tornado forming, the likelihood of the politician’s reelection, or whatever you like. The concept of a function is central to mathematics and to math’s ubiquity in the world.
The most fundamental example of a function is a polynomial. Everyone sees polynomials in school. These are the functions whose rules involve taking the input to powers, perhaps multiplying by numbers, and perhaps doing some additions and subtractions. For example,
Here the input variable is x. When you input x= 2 or x=1 or x=-1, you get p(2)=3, p(1)=2, and p(-1)=6 as outputs.
You may recall that a zero (or root) of a polynomial is the name for an input that happens to give you zero as an output. For example, the polynomial x² – 4x + 3 has zeros x=1 and x=3. On the other hand, the polynomial p(x) doesn’t seem to have any zeros.
Many applications in and out of math come down to the question of finding the zeros of a polynomial. Several years ago, we talked here at 3QD about how certain geometric constructions from Ancient Greece are impossible with ruler and compass, but become possible using origami. Under the hood, this came down to understanding the zeros of certain polynomials. Important parts of number theory and geometry are essentially about the problem of understanding the zeros of polynomials. Read more »