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Logarithmic form : ウィキペディア英語版
Logarithmic form
In contexts including complex manifolds and algebraic geometry, a logarithmic differential form is a meromorphic differential form with poles of a certain kind.
Let ''X'' be a complex manifold, and ''D'' ⊂ ''X'' a divisor and ω a holomorphic ''p''-form on ''X''−''D''. If ω and ''d''ω have a pole of order at most one along ''D'', then ω is said to have a logarithmic pole along ''D''. ω is also known as a logarithmic ''p''-form. The logarithmic ''p''-forms make up a subsheaf of the meromorphic ''p''-forms on ''X'' with a pole along ''D'', denoted
:\Omega^p_X(\log D).
In the theory of Riemann surfaces, one encounters logarithmic one-forms which have the local expression
:\omega = \frac =\left(\frac + \frac\right)dz
for some meromorphic function (resp. rational function) f(z) = z^mg(z) , where ''g'' is holomorphic and non-vanishing at 0, and ''m'' is the order of ''f'' at ''0''. That is, for some open covering, there are local representations of this differential form as a logarithmic derivative (modified slightly with the exterior derivative ''d'' in place of the usual differential operator ''d/dz''). Observe that ω has only simple poles with integer residues. On higher-dimensional complex manifolds, the Poincaré residue is used to describe the distinctive behavior of logarithmic forms along poles.
==Holomorphic log complex==
By definition of \Omega^p_X(\log D) and the fact that exterior differentiation ''d'' satisfies ''d''2 = 0, one has
: d\Omega^p_X(\log D)(U)\subset \Omega^_X(\log D)(U) .
This implies that there is a complex of sheaves ( \Omega^_X(\log D), d) , known as the ''holomorphic log complex'' corresponding to the divisor ''D''. This is a subcomplex of j_
*\Omega^_ , where j:X-D\rightarrow X is the inclusion and \Omega^_ is the complex of sheaves of holomorphic forms on ''X''−''D''.
Of special interest is the case where ''D'' has simple normal crossings. Then if \ are the smooth, irreducible components of ''D'', one has D = \sum D_ with the D_ meeting transversely. Locally ''D'' is the union of hyperplanes, with local defining equations of the form z_1\cdots z_k = 0 in some holomorphic coordinates. One can show that the stalk of \Omega^1_X(\log D) at ''p'' satisfies〔Chris A.M. Peters; Joseph H.M. Steenbrink (2007). Mixed Hodge Structures. Springer. ISBN 978-3-540-77017-6〕
:\Omega_X^1(\log D)_p = \mathcal_\frac\oplus\cdots\oplus\mathcal_\frac \oplus \mathcal_dz_ \oplus \cdots \oplus \mathcal_dz_n
and that
: \Omega_X^k(\log D)_p = \bigwedge^k_ \Omega_X^1(\log D)_p .
Some authors, e.g.,〔Phillip A. Griffiths; Joseph Harris (1979). Principles of Algebraic Geometry. Wiley-Interscience. ISBN 0-471-05059-8.〕 use the term ''log complex'' to refer to the holomorphic log complex corresponding to a divisor with normal crossings.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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