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Cartier divisor (Definition)

On a scheme $ X$, a Cartier divisor is a global section of the sheaf $ \mathcal{K}^*/\O ^*$, where $ \mathcal{K}^*$ is the multiplicative sheaf of meromorphic functions, and $ \O ^*$ the multiplicative sheaf of invertible regular functions (the units of the structure sheaf).

More explicitly, a Cartier divisor is a choice of open cover $ U_i$ of $ X$, and meromorphic functions $ f_i\in \mathcal{K}^*(U_i)$, such that $ f_i/f_j\in\O ^*(U_{i}\cap U_j)$, along with two Cartier divisors being the same if the open cover of one is a refinement of the other, with the same functions attached to open sets, or if $ f_i$ is replaced by $ gf_i$ with $ g\in\O _*$.

Intuitively, the only information carried by Cartier divisor is where it vanishes, and the order it does there. Thus, a Cartier divisor should give us a Weil divisor, and vice versa. On “nice” (for example, nonsingular over an algebraically closed field) schemes, it does.



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Cross-references: field, algebraically closed, nonsingular, Weil divisor, order, vanishes, open sets, refinement, functions, meromorphic, open cover, structure sheaf, units, regular functions, invertible, sheaf of meromorphic functions, multiplicative, sheaf, global section, scheme
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This is version 3 of Cartier divisor, born on 2003-08-19, modified 2004-10-25.
Object id is 4617, canonical name is CartierDivisor.
Accessed 3453 times total.

Classification:
AMS MSC14A99 (Algebraic geometry :: Foundations :: Miscellaneous)

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