2 1. Warming up to Enumerative Geometry “How many points in the plane lie on each of two given lines?” However, even this simple example reveals several potential diﬃ- culties of a type which we will have to frequently address: • The answer depends on the choice of lines: (1) If the lines are chosen generally, there is exactly one such point, the point of intersection of the two lines. (2) The point can “disappear to infinity” if the lines are moved into a parallel position, in a precise way to be explained shortly. (3) The answer can be infinite (if the two lines coincide with each other). • Note as well that the question can be translated into a ques- tion of algebra: how many solutions (x, y) are there to the system of equations ax + by = c, dx + ey = f? This somewhat cumbersome answer to a very simple question is unpleasant. In enumerative geometry, this is dealt with by changing the question slightly, sacrificing the simplicity of the question in favor of simplicity of the answer, in this case an unequivocal “1”. Let’s shift from the context of geometry to the context of algebra, as we just saw we can do in this example.1 Let’s stay in the realm of the elementary and ask the enumerative (algebraic) question: “how many roots does a polynomial of degree n in one variable have?” For this problem, there is again a range of subcases, revealing a range of potential diﬃculties. Let’s consider a polynomial of degree d f(x) = a0xd + a1xd−1 + ··· + ad−1x + ad, for the moment with real coeﬃcients ai, and solve f(x) = 0. Let’s first consider d = 1. The solution is x = −a1/a0, and there is at first glance exactly one solution. There is a diﬃculty, which we will see is more than a simple issue of semantics: suppose we agree to consider f(x) = 0 · x + a1 as a degenerate degree 1 polynomial after all, f(x) 1 This ability to move freely from geometry to algebra and back is at the core of the subject called algebraic geometry.

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