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In [[algebraic geometry]] and the theory of [[complex manifold]]s, a '''logarithmic''' [[differential form]] is a differential form with [[pole (complex analysis)|poles]] of a certain kind. The concept was introduced by [[Pierre Deligne]].<ref>Deligne (1970), section II.3.</ref> In short, logarithmic differentials have the mildest possible singularities needed in order to give information about an open [[submanifold]] (the complement of the divisor of poles). (This idea is made precise by several versions of [[de Rham's theorem]] discussed below.)
Let ''X'' be a complex manifold, ''D'' ⊂ ''X'' a reduced [[Divisor (algebraic geometry)|divisor]] (a sum of distinct codimension-1 complex subspaces), and ω a holomorphic ''p''-form on ''X''−''D''. If both ω and ''d''ω have a pole of order at most 1 along ''D'', then ω is said to have a logarithmic pole along ''D''. ω is also known as a logarithmic ''p''-form. The ''p''-forms with log poles along ''D'' form a [[Sheaf (mathematics)|subsheaf]] of the meromorphic ''p''-forms on ''X'', denoted
:<math>\Omega^p_X(\log D).</math>
The name comes from the fact that in [[complex analysis]], <math>d(\log z)=dz/z</math>; here <math>dz/z</math> is a typical example of a 1-form on the [[complex
==Logarithmic de Rham complex==
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There is an [[exact sequence]] of [[coherent sheaves]] on ''X'':
:<math>0 \to \Omega^1_X \to \Omega^1_X(\log D) \overset{\beta}\to \oplus_j ({i_j})_*\mathcal{O}_{D_j} \to 0,</math>
where <math>i_j: D_j \to X</math> is the inclusion of an irreducible component of ''D''. Here β is called the '''residue''' map; so this sequence says that a 1-form with log poles along ''D'' is regular (that is, has no poles) [[if and only if]] its residues are zero. More generally, for any ''p'' ≥ 0, there is an exact sequence of coherent sheaves on ''X'':
: <math>0 \to \Omega^p_X \to \Omega^p_X(\log D) \overset{\beta}\to \oplus_j ({i_j})_*\Omega^{p-1}_{D_j}(\log (D-D_j)) \to \cdots \to 0,</math>
where the sums run over all irreducible components of given dimension of intersections of the divisors ''D''<sub>''j''</sub>. Here again, β is called the residue map.
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==Mixed Hodge theory for smooth varieties==
Over the complex numbers, Deligne proved a strengthening of [[Alexander Grothendieck]]'s algebraic [[de Rham theorem]], relating [[coherent sheaf cohomology]] with [[singular cohomology]]. Namely, for any smooth scheme ''X'' over '''C''' with a divisor with simple normal crossings ''D'', there is a natural isomorphism
:<math> H^k(X, \Omega^{\bullet}_X(\log D)) \cong H^k(X-D,\mathbf{C})</math>
for each integer ''k'', where the groups on the left are defined using the [[Zariski topology]] and the groups on the right use the classical (Euclidean) topology.<ref>Deligne (1970), Corollaire II.6.10.</ref>
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degenerates at <math>E_1</math>.<ref>Deligne (1971), Corollaire 3.2.13.</ref> So the cohomology of <math>X-D</math> with complex coefficients has a decreasing filtration, the '''Hodge filtration''', whose associated graded vector spaces are the algebraically defined groups <math>H^q(X,\Omega^p_X(\log D))</math>.
This is part of the [[mixed Hodge structure]] which Deligne defined on the cohomology of any [[complex algebraic variety]]. In particular, there is also a '''weight filtration''' on the rational cohomology of <math>X-D</math>. The resulting filtration on <math>H^*(X-D,\mathbf{C})</math> can be constructed using the logarithmic de Rham complex. Namely, define an increasing filtration <math>W_{\bullet} \Omega^p_X(\log D) </math> by
:<math>W_{m}\Omega^p_X(\log D) = \begin{cases}
0 & m < 0\\
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:<math> W_mH^k(X-D, \mathbf{C}) = \text{Im}(H^k(X, W_{m-k}\Omega^{\bullet}_X(\log D))\rightarrow H^k(X-D,\mathbf{C})).</math>
Building on these results, [[Hélène Esnault]] and [[Eckart Viehweg]] generalized the [[Nakano vanishing theorem|Kodaira–Akizuki–Nakano vanishing theorem]] in terms of logarithmic differentials. Namely, let ''X'' be a smooth complex [[projective variety]] of dimension ''n'', ''D'' a divisor with simple normal crossings on ''X'', and ''L'' an [[ample line bundle]] on ''X''. Then
:<math>H^q(X,\Omega^p_X(\log D)\otimes L)=0</math>
and
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==References==
* {{Citation | last1=Deligne | first1=Pierre |
* {{citation|author1-last=Deligne|author1-first=Pierre|author1-link=Pierre Deligne|title=Théorie de Hodge II|journal=Publ. Math. IHÉS |volume=40|pages=5–57|year=1971|doi=10.1007/BF02684692 |mr=0498551|s2cid=118967613 |url=http://www.numdam.org/item/PMIHES_1971__40__5_0/|url-access=subscription}}
*{{Citation|author1-last=Esnault|author1-first=Hélène | author1-link=Hélène Esnault | author2-last=Viehweg | author2-first=Eckart | author2-link=Eckart Viehweg | title=Lectures on vanishing theorems | publisher=Birkhäuser| isbn=978-3-7643-2822-1 |mr=1193913 | year=1992|doi=10.1007/978-3-0348-8600-0}}
*{{citation|last1=Griffiths |first1=Phillip | author-link1=Phillip Griffiths |last2=Harris |first2=Joseph |author-link2=Joe Harris (mathematician) | title=Principles of algebraic geometry | series=Wiley Classics Library | publisher=Wiley Interscience | year=1994 | orig-year=1978 | isbn=0-471-05059-8 | mr=0507725|doi=10.1002/9781118032527}}
* {{citation|author1-last=Peters|author1-first=Chris A.M.|author2-last=Steenbrink|author2-first=Joseph H. M.|author2-link=Joseph H. M. Steenbrink|title=Mixed Hodge structures|series=Ergebnisse der Mathematik und ihrer Grenzgebiete. 3. Folge / A Series of Modern Surveys in Mathematics |publisher=Springer|year=2008|volume=52 |isbn=978-3-540-77017-6|mr=2393625|doi=10.1007/978-3-540-77017-6}}
==External links==
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