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| In [[mathematics]], the '''theorem of Bertini''' is an existence and genericity theorem for smooth connected [[hyperplane section]]s for smooth projective varieties over [[algebraically closed field]]s, introduced by [[Eugenio Bertini]]. This is the simplest and broadest of the "Bertini theorems" applying to a [[linear system of divisors]]; simplest because there is no restriction on the [[characteristic of a field|characteristic]] of the underlying field, while the extensions require characteristic 0.<ref>{{Springer|id=B/b015770|title=Bertini theorems}}</ref><ref>Hartshorne, Ch. III.10.</ref>
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| == Statement for hyperplane sections of smooth varieties==
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| Let ''X'' be a smooth quasi-projective variety over an algebraically closed field, embedded in a [[algebraic geometry of projective spaces|projective space]] <math>\mathbf P^n</math>.
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| Let <math>|H|</math> denote the [[linear system of divisors|complete system]] of hyperplane divisors in <math>\mathbf P^n</math>. Recall that it is the [[Duality_(projective_geometry)#Higher dimensional duality|dual space]] <math>(\mathbf P^n)^{\star}</math> of <math>\mathbf P^n</math> and is isomorphic to <math>\mathbf P^n</math>.
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| The theorem of Bertini states that the set of hyperplanes not containing ''X'' and with smooth intersection with ''X'' contains an open dense subset of the total system of divisors <math>|H|</math>. The set itself is open if ''X'' is projective. If dim(''X'') ≥ 2, then these intersections (called hyperplane sections of ''X'') are connected, hence irreducible.
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| The theorem hence asserts that a ''general'' hyperplane section not equal to ''X'' is smooth, that is: the property of smoothness is generic.
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| Over an arbitrary field ''k'', there is a dense open subset of the dual space <math>(\mathbf P^n)^{\star}</math> whose [[rational point]]s define hyperplanes smooth hyperplane sections of ''X''. When ''k'' is infinite, this open subset then has infinitely many rational points and there are infinitely many smooth hyperplane sections in ''X''.
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| Over a finite field, the above open subset may not contain rational points and in general there is no hyperplanes with smooth intersection with ''X''. However, if we take hypersurfaces of sufficientely big degrees, then the theorem of Bertini holds.<ref>Bjorn Poonen: [http://math.mit.edu/~poonen/papers/bertini.pdf ''Bertini Theorems over finite fields''], Ann. of Math. '''160''' (2004).</ref>
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| ==Outline of a proof==
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| We consider the subfibration of the product variety <math>X \times |H|</math> with fiber above <math>x\in X</math> the linear system of hyperplanes that intersect ''X'' non-[[transversality (mathematics)|transversally]] at ''x''.
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| The rank of the fibration in the product is one less than the codimension of <math>X \subset \mathbf P^n</math>, so that the total space has lesser dimension than <math>n</math> and so its projection is contained in a divisor of the complete system <math>|H|</math>.
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| ==General statement==
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| Over <math>\mathbb C</math> (or an algebraically closed field of characteristic 0), if ''X'' is a smooth quasi-projective variety, a general member of a [[linear system of divisors]] on ''X'' is smooth away from the [[base locus]] of the system. This property fails in positive characteristics.
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| == Generalizations ==
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| The theorem of Bertini has been generalized in various ways. For example, a result due to [[Steven Kleiman]] asserts the following: for a connected [[algebraic group]] ''G'', and any homogeneous ''G''-variety ''X'', and two varieties ''Y'' and ''Z'' mapping to ''X'', let ''Y''<sup>σ</sup> be the variety obtained by letting σ ∈ ''G'' act on ''Y''. Then, there is an open dense subscheme ''H'' of ''G'' such that for σ ∈ ''H'', <math>Y^\sigma \times_X Z</math> is either empty or purely of the (expected) [[Krull dimension|dimension]] dim ''Y'' + dim ''Z'' − dim ''X''. If, in addition, ''Y'' and ''Z'' are [[smooth scheme|smooth]], then ''H'' may be taken such that <math>Y^\sigma \times_X Z</math> is smooth for all <math>\sigma \in H</math>, as well. The above theorem of Bertini is the special case where <math>X = \mathbb P^n</math> is expressed as the quotient of SL<sub>''n''</sub> by the [[parabolic subgroup]] of upper triangular matrices, ''Z'' is a subvariety and ''Y'' is a hyperplane.<ref>{{Citation | last1=Kleiman | first1=Steven L. | author1-link=Steven Kleiman | title=The transversality of a general translate | year=1974 | journal=Compositio Mathematica | issn=0010-437X | volume=28 | pages=287–297}}</ref>
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| Theorem of Bertini has also been generalized to discrete valuation domains or finite fields, or for étale coverings of ''X''.
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| The theorem is often used for induction steps.
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| == Notes ==
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| {{Reflist}}
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| == References ==
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| * {{Hartshorne AG}}
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| *[http://arxiv.org/pdf/alg-geom/9704018v1 Bertini and his two fundamental theorems] by Steven L. Kleiman, on the life and works of [[Eugenio Bertini]]
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| [[Category:Geometry of divisors]]
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| [[Category:Theorems in algebraic geometry]]
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My name is Tayla and I am studying Physical and Agriculture and Life Sciences at Sinagra / Australia.
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