Who Is Alexander Grothendieck? (2008) [pdf]

Re:Who Is Alexander Grothendieck? (2008) [pdf] by eafer on Monday August 03, 2020 @01:34PM (#24040785)

> Concentric circles, e.g. x^2 + y^2 = 1 and x^2 + y^2 = 4. They don't intersect at all, right? Ah, except you forgot to count points over the complex numbers, where they do.

How is this possible? Are you saying that there are pairs of complex numbers (x,y) such that x^2 + y^2 = 1 = 4?

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Re:Who Is Alexander Grothendieck? (2008) [pdf] by alcolade on Monday August 03, 2020 @02:59PM (#24041879)

You need to also count "points at infinity" in the "projective" plane. So the "projectivized" equations are actually X^2 + Y^2 = 1Z, and X^2 + Y^2 = 4Z. The intuition is similar to how 2 parallel lines will meet at the horizion (infinity).

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Re:Who Is Alexander Grothendieck? (2008) [pdf] by eafer on Monday August 03, 2020 @05:41PM (#24043793)

Thanks, I think I got it.

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Re:Who Is Alexander Grothendieck? (2008) [pdf] by impendia on Monday August 03, 2020 @04:48PM (#24043211)

Oops, brain fart. No, 1 is not equal to 4 even in Grothendieck's world. Scheme theory only gets you so far...

A correct example of what I had in mind is x^2 + y^2 = 1 and (x - 2)^2 + y^2 = 1.

That said, algebraic geometry takes care of this "bad example" also. This is done exactly as alcolade explained: consider "points at infinity" in projective space.

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