A Question about intersection of coordinate rings

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The discussion focuses on proving that the coordinate ring of the affine space minus the origin, ##\mathcal{O}_{\mathbb{A}^n}(\mathbb{A}^n \setminus \{0\})##, equals ##k[X_1, \ldots, X_n]## for ##n \ge 2##. A user outlines a proof involving the intersection of rings of regular functions on the open sets defined by the vanishing of coordinates, specifically using the notation ##D(X_i)##. Clarifications are provided regarding the nature of these rings, emphasizing that ##\mathcal{O}(D(X_i))## represents localizations rather than quotients, allowing for the intersection of these rings. The distinction between regular functions and ideals is highlighted, reinforcing that the intersection of these function rings remains within the field of rational functions. The conversation ultimately clarifies the correct interpretation of the notation and the underlying algebraic structures.
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I want to prove that ##\mathcal{O}_{\mathbb{A}^n}(\mathbb{A}^n \setminus \{0\}) = k[X_1, \ldots, X_n]## for ##n \ge 2## and some user on another forum gave this proof:

"We know that ##\mathcal{O}_{\mathbb{A}^n}(\mathbb{A}^n \setminus \{0\}) = \bigcap_{i = 1}^n \mathcal{O}(D(X_i))##, where ##D(f) = \{x \in \mathbb{A}^n : f(x) \ne 0\}##. Now ##\mathcal{O}(D(X_i)) = k[X_1, \ldots,X_n,\frac{1}{X_i}]## and ##\mathcal{O}(D(X_i)) \bigcap \mathcal{O}(D(X_j)) = k[X_1, \ldots, X_n]## for every ##i \ne j##.

This implies that ##\bigcap_{i = 1}^n \mathcal{O}(D(X_i)) = k[X_1,\ldots,X_n]##, so ##\mathcal{O}_{\mathbb{A}^n}(\mathbb{A}^n \setminus \{0\}) = k[X_1,\ldots, X_n]##."

I don't understand some notations. I know that ##\mathbb{A}^n \setminus\{0\} = \bigcup_{i=1}^n D(X_i)## and that ##\mathcal{O}_{\mathbb{A}^n}(\mathbb{A}^n \setminus \{0\}) = \{f : \mathbb{A}^n \setminus \{0\} \to k : f \text{ is regular on U}\}## and for every ##i \in \{1, 2, \ldots, n\}## we have ##\mathcal{O}(D(X_i)) = k[X_1,\ldots,X_n]/(I(X_i))## where ##I(X_i) = \{f \in k[X_1,\ldots,X_n] : f(x) = 0 \text{ for any } x \in (X_i)\}##. Now I don't understand why we can intersect those quotient rings. I know that we can interpret ##\mathcal{O}(D(X_i))## as restrictions of polynomials on ##D(X_i)## but I still don't understand how we can intersect those terms. (And, shouldn't be ##\mathcal{O}_{\mathbb{A}^n}(D(X_i))## instead of ##\mathcal{O}(D(X_i))##?)
 
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##D(f)## is the set on which ##f## is not zero, and ##\mathcal O(D(f))## is the ring of regular functions on that set. It is not a quotient, it is a localization. You can have any powers of ##f## in the denominator. You are confusing them with the zero of an ideal and the regular functions on it. This is why ##\mathcal O(D(X_i))## is ##k[X_1,X_2,\dots,X_n,\frac1{X_i}]## and not the quotient you think. All of these are subrings of the field of rational functions and the intersection is inside it.
 
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