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(Weil divisors : Cartier divisors) = (p-Cycles : ? )

Posted on 11/1/2012
UQ ID:336
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Mathematics
Original

Suppose verifies the suitable conditions in which Weil (resp. Cartier) divisors make sense. The group of Weil divisors on a scheme is the free abelian group generated by codimention irreducible subvarieties. The sheaf of Cartier divisors is . The group of Cartier divisors is . The group of -cycles is the free abelian group generated by irreducible subvarieties of codimension , so in particular . So the notion of -cycle is a direct generalization of the notion of Weil divisor. My question:

Is there an analogous notion of group of "Cartier -cycles" ? If yes, is there a sheaf such that (naturally in ) we have ?

Keywords & Tags
algebraic-geometry
intersection-theory
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K2-Think

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Qwen2.5-32B
Sep 18, 2025, 01:50 PM

Yes, there is an analogous notion of "Cartier -cycles" and a corresponding sheaf.

Key Idea:

For , Cartier divisors are defined using the quotient sheaf , where is the sheaf of total quotient rings (rational functions) and is the sheaf of invertible regular functions. This sheaf's global sections $\Gamma...

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GPT-5

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OpenAI
Sep 2, 2025, 06:56 AM

Short answer: not as a single sheaf. The right higher‑codimension replacement of “Cartier divisors” is no longer captured by global sections of one sheaf; instead one needs a (Gersten) complex of sheaves coming from K‑theory whose last term is the sheaf of codimension‑p cycles.

More precisely (assume X is regular, noetherian; e.g. a smooth variety), set Z^p_X := ⊕{x∈X^{(p)}} i{x,*} Z, the sheaf...

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o3 Pro

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OpenAI
Aug 1, 2025, 12:00 AM

Let us recall what makes a Cartier divisor different from a Weil divisor.

• A Weil divisor is just a formal linear combination of codimension-1 subvarieties.
• A Cartier divisor, on the other hand, is a subvariety that can be cut out by ONE equation – i.e. its ideal sheaf is locally generated by a single non–zero‐divisor. “Locally generated by one element” is what is measured by the sheaf
...

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Claude Opus 4

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Anthropic
Aug 1, 2025, 12:00 AM

Yes, there is a notion of higher codimension Cartier cycles, though the theory becomes more subtle and less universally applicable than for divisors. Let me explain the main approaches.

The Analogue: Higher Chow Groups and K-theory

The most direct analogue of Cartier divisors for higher codimension cycles comes from Milnor K-theory and higher Chow groups.

1. Milnor K-sheaves

For...

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Gemini 2.5 Pro

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Google
Aug 1, 2025, 12:00 AM

This is an excellent and very natural question. The transition from Weil divisors to Cartier divisors is a fundamental step in algebraic geometry, moving from a purely geometric definition (formal sums of subvarieties) to one that is more sheaf-theoretic and tied to local equations. You are asking for the analogous step in higher codimension.

The short answer is: **Yes, such notions exist, but th...

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o3

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OpenAI
Aug 1, 2025, 12:00 AM

Only in codimension 1 do “Cartier objects’’ behave at all like Weil cycles.
The reason is essentially algebraic: a closed subscheme of codimension 1 can be described (étale-locally, or even Zariski-locally if the scheme is integral and normal) by a single non-zero divisor (i.e. by one equation), and two such local equations differ by a unit. Therefore the transition functions live in 𝒪×, an ab...

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Claude 3.7 Sonnet

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Anthropic
Aug 1, 2025, 12:00 AM

Generalizing Cartier Divisors to Higher Codimensions

The question asks about finding a natural generalization of the relationship between Weil divisors and Cartier divisors to higher codimensions.

The Existing Relationship

For codimension 1:

  • Weil divisors (Div(X)) are formal sums of codimension 1 subvarieties
  • Cartier divisors (Cart(X)) are global sections of $\mathcal{K}_X^{\times}/\ma...
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DeepSeek R1

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DeepSeek
deepseek-ai/DeepSeek-R1
Aug 1, 2025, 12:00 AM
<think> We are given a question about generalizing the relationship between Weil divisors and Cartier divisors to higher codimension. Specifically, we know that for codimension 1, Weil divisors correspond to Cartier divisors under suitable conditions. The question asks if there is an analogous notion of "Cartier p-cycles" for codimension p, and if so, whether it can be defined as the global sectio...
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o4 mini

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OpenAI
Aug 1, 2025, 12:00 AM

Here is one reasonably concrete way to see that “Cartier‐p–cycles’’ exist exactly by imitating the case p=1, but replacing K₁ by Quillen’s higher K‐theory sheaves. Recall that for p=1 one has on the Zariski site of X the exact sequence of sheaves

0 → 𝒪_X^× → 𝒦_X^× → Cart_X^1 → 0

and by definition Cart^1(X) = Γ(X,Cart_X^1) = coker[Γ(X,𝒪_X^×)→Γ(X,𝒦_X^×)] = principal Weil‐div...

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