Any hyperplanes is covered by non-Lefschetz pencils?











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Let $Xsubset mathbb P^n$ be a smooth hypersurface over base field $mathbb C$. A pencil of hyperplanes is just a projective line $(X_t)$ in $mathbb P^{n*}$. It is called a Lefschetz pencil if it satisfies the following: (I followed the definition in SGA)



(i) The axis (intersection of $X_t$'s) intersects with $X$ transversally.



(ii) Almost all (enough for one) $X_t$ intersects with $X$ transversally.



(iii) The non-transversal intersection in (ii) contains one node.



And on SGA it also proves that almost all pencils are Lefschetz. Now I want to know how many are those non-Lefschetz pencils, more precisely:




Does there exist some $X$, on which all the non-Lefschetz pencils cover the $mathbb P^{n*}$? i.e. for any hyperplane section $H$, we can find non-Lefschetz pencil $(X_t)$ pass through it.




I guess it does not exist, but have no idea how to prove it. Any hints or references would be helpful.



Remark



I should remove the condition (i) otherwise it is trivial.










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    up vote
    2
    down vote

    favorite
    1












    Let $Xsubset mathbb P^n$ be a smooth hypersurface over base field $mathbb C$. A pencil of hyperplanes is just a projective line $(X_t)$ in $mathbb P^{n*}$. It is called a Lefschetz pencil if it satisfies the following: (I followed the definition in SGA)



    (i) The axis (intersection of $X_t$'s) intersects with $X$ transversally.



    (ii) Almost all (enough for one) $X_t$ intersects with $X$ transversally.



    (iii) The non-transversal intersection in (ii) contains one node.



    And on SGA it also proves that almost all pencils are Lefschetz. Now I want to know how many are those non-Lefschetz pencils, more precisely:




    Does there exist some $X$, on which all the non-Lefschetz pencils cover the $mathbb P^{n*}$? i.e. for any hyperplane section $H$, we can find non-Lefschetz pencil $(X_t)$ pass through it.




    I guess it does not exist, but have no idea how to prove it. Any hints or references would be helpful.



    Remark



    I should remove the condition (i) otherwise it is trivial.










    share|cite|improve this question


























      up vote
      2
      down vote

      favorite
      1









      up vote
      2
      down vote

      favorite
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      1





      Let $Xsubset mathbb P^n$ be a smooth hypersurface over base field $mathbb C$. A pencil of hyperplanes is just a projective line $(X_t)$ in $mathbb P^{n*}$. It is called a Lefschetz pencil if it satisfies the following: (I followed the definition in SGA)



      (i) The axis (intersection of $X_t$'s) intersects with $X$ transversally.



      (ii) Almost all (enough for one) $X_t$ intersects with $X$ transversally.



      (iii) The non-transversal intersection in (ii) contains one node.



      And on SGA it also proves that almost all pencils are Lefschetz. Now I want to know how many are those non-Lefschetz pencils, more precisely:




      Does there exist some $X$, on which all the non-Lefschetz pencils cover the $mathbb P^{n*}$? i.e. for any hyperplane section $H$, we can find non-Lefschetz pencil $(X_t)$ pass through it.




      I guess it does not exist, but have no idea how to prove it. Any hints or references would be helpful.



      Remark



      I should remove the condition (i) otherwise it is trivial.










      share|cite|improve this question















      Let $Xsubset mathbb P^n$ be a smooth hypersurface over base field $mathbb C$. A pencil of hyperplanes is just a projective line $(X_t)$ in $mathbb P^{n*}$. It is called a Lefschetz pencil if it satisfies the following: (I followed the definition in SGA)



      (i) The axis (intersection of $X_t$'s) intersects with $X$ transversally.



      (ii) Almost all (enough for one) $X_t$ intersects with $X$ transversally.



      (iii) The non-transversal intersection in (ii) contains one node.



      And on SGA it also proves that almost all pencils are Lefschetz. Now I want to know how many are those non-Lefschetz pencils, more precisely:




      Does there exist some $X$, on which all the non-Lefschetz pencils cover the $mathbb P^{n*}$? i.e. for any hyperplane section $H$, we can find non-Lefschetz pencil $(X_t)$ pass through it.




      I guess it does not exist, but have no idea how to prove it. Any hints or references would be helpful.



      Remark



      I should remove the condition (i) otherwise it is trivial.







      algebraic-geometry complex-geometry intersection-theory






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      edited Nov 16 at 22:30

























      asked Nov 16 at 21:41









      User X

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          Assume there is a single hyperplane section $X_0$ of $X$ which has worse singularity than just a node. Then any pencil containing $X_0$ is non-Lefschetz, and these pencils cover all the $mathbb{P}^{n*}$.



          Of course, if there is no such $X_0$ as above, this construction does not work. However, I think the only case when such $X_0$ does not exist is that of $X$ being a smooth quadric. In that case the projective dual variety to $X$ is yet another smooth quadric, and lines in $mathbb{P}^{n*}$ tangent to it provide non-Lefschetz pencils that cover all the $mathbb{P}^{n*}$.






          share|cite|improve this answer





















          • Thanks for your answer. Btw can I ask another question (maybe I should start a new question but anyway) why we need the condition (i) in the definition? Isn’t it followed by (ii)?
            – User X
            Nov 17 at 9:39










          • Please, give a precise reference to the definition.
            – Sasha
            Nov 17 at 10:12










          • I followed the definition in Groupes de monodromie en géométrie algébrique. II. Lecture Notes in Mathematics, Vol. 340. Séminaire de Géométrie Algébrique du Bois-Marie 1967–1969 (SGA 7 II).
            – User X
            Nov 17 at 11:20












          • A precise reference should include expose and section numbers. In fact "axis" ("l'axe") in English is usually called "base locus". It surely doesn't follow from (ii). For instance, take $X subset mathbb{P}^2$ to be a smooth conic, and the pencil of lines through a point ON $X$.
            – Sasha
            Nov 17 at 14:44










          • Sorry I thought something wrong. Thanks.
            – User X
            Nov 17 at 19:07











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          up vote
          2
          down vote



          accepted










          Assume there is a single hyperplane section $X_0$ of $X$ which has worse singularity than just a node. Then any pencil containing $X_0$ is non-Lefschetz, and these pencils cover all the $mathbb{P}^{n*}$.



          Of course, if there is no such $X_0$ as above, this construction does not work. However, I think the only case when such $X_0$ does not exist is that of $X$ being a smooth quadric. In that case the projective dual variety to $X$ is yet another smooth quadric, and lines in $mathbb{P}^{n*}$ tangent to it provide non-Lefschetz pencils that cover all the $mathbb{P}^{n*}$.






          share|cite|improve this answer





















          • Thanks for your answer. Btw can I ask another question (maybe I should start a new question but anyway) why we need the condition (i) in the definition? Isn’t it followed by (ii)?
            – User X
            Nov 17 at 9:39










          • Please, give a precise reference to the definition.
            – Sasha
            Nov 17 at 10:12










          • I followed the definition in Groupes de monodromie en géométrie algébrique. II. Lecture Notes in Mathematics, Vol. 340. Séminaire de Géométrie Algébrique du Bois-Marie 1967–1969 (SGA 7 II).
            – User X
            Nov 17 at 11:20












          • A precise reference should include expose and section numbers. In fact "axis" ("l'axe") in English is usually called "base locus". It surely doesn't follow from (ii). For instance, take $X subset mathbb{P}^2$ to be a smooth conic, and the pencil of lines through a point ON $X$.
            – Sasha
            Nov 17 at 14:44










          • Sorry I thought something wrong. Thanks.
            – User X
            Nov 17 at 19:07















          up vote
          2
          down vote



          accepted










          Assume there is a single hyperplane section $X_0$ of $X$ which has worse singularity than just a node. Then any pencil containing $X_0$ is non-Lefschetz, and these pencils cover all the $mathbb{P}^{n*}$.



          Of course, if there is no such $X_0$ as above, this construction does not work. However, I think the only case when such $X_0$ does not exist is that of $X$ being a smooth quadric. In that case the projective dual variety to $X$ is yet another smooth quadric, and lines in $mathbb{P}^{n*}$ tangent to it provide non-Lefschetz pencils that cover all the $mathbb{P}^{n*}$.






          share|cite|improve this answer





















          • Thanks for your answer. Btw can I ask another question (maybe I should start a new question but anyway) why we need the condition (i) in the definition? Isn’t it followed by (ii)?
            – User X
            Nov 17 at 9:39










          • Please, give a precise reference to the definition.
            – Sasha
            Nov 17 at 10:12










          • I followed the definition in Groupes de monodromie en géométrie algébrique. II. Lecture Notes in Mathematics, Vol. 340. Séminaire de Géométrie Algébrique du Bois-Marie 1967–1969 (SGA 7 II).
            – User X
            Nov 17 at 11:20












          • A precise reference should include expose and section numbers. In fact "axis" ("l'axe") in English is usually called "base locus". It surely doesn't follow from (ii). For instance, take $X subset mathbb{P}^2$ to be a smooth conic, and the pencil of lines through a point ON $X$.
            – Sasha
            Nov 17 at 14:44










          • Sorry I thought something wrong. Thanks.
            – User X
            Nov 17 at 19:07













          up vote
          2
          down vote



          accepted







          up vote
          2
          down vote



          accepted






          Assume there is a single hyperplane section $X_0$ of $X$ which has worse singularity than just a node. Then any pencil containing $X_0$ is non-Lefschetz, and these pencils cover all the $mathbb{P}^{n*}$.



          Of course, if there is no such $X_0$ as above, this construction does not work. However, I think the only case when such $X_0$ does not exist is that of $X$ being a smooth quadric. In that case the projective dual variety to $X$ is yet another smooth quadric, and lines in $mathbb{P}^{n*}$ tangent to it provide non-Lefschetz pencils that cover all the $mathbb{P}^{n*}$.






          share|cite|improve this answer












          Assume there is a single hyperplane section $X_0$ of $X$ which has worse singularity than just a node. Then any pencil containing $X_0$ is non-Lefschetz, and these pencils cover all the $mathbb{P}^{n*}$.



          Of course, if there is no such $X_0$ as above, this construction does not work. However, I think the only case when such $X_0$ does not exist is that of $X$ being a smooth quadric. In that case the projective dual variety to $X$ is yet another smooth quadric, and lines in $mathbb{P}^{n*}$ tangent to it provide non-Lefschetz pencils that cover all the $mathbb{P}^{n*}$.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Nov 17 at 7:54









          Sasha

          4,07339




          4,07339












          • Thanks for your answer. Btw can I ask another question (maybe I should start a new question but anyway) why we need the condition (i) in the definition? Isn’t it followed by (ii)?
            – User X
            Nov 17 at 9:39










          • Please, give a precise reference to the definition.
            – Sasha
            Nov 17 at 10:12










          • I followed the definition in Groupes de monodromie en géométrie algébrique. II. Lecture Notes in Mathematics, Vol. 340. Séminaire de Géométrie Algébrique du Bois-Marie 1967–1969 (SGA 7 II).
            – User X
            Nov 17 at 11:20












          • A precise reference should include expose and section numbers. In fact "axis" ("l'axe") in English is usually called "base locus". It surely doesn't follow from (ii). For instance, take $X subset mathbb{P}^2$ to be a smooth conic, and the pencil of lines through a point ON $X$.
            – Sasha
            Nov 17 at 14:44










          • Sorry I thought something wrong. Thanks.
            – User X
            Nov 17 at 19:07


















          • Thanks for your answer. Btw can I ask another question (maybe I should start a new question but anyway) why we need the condition (i) in the definition? Isn’t it followed by (ii)?
            – User X
            Nov 17 at 9:39










          • Please, give a precise reference to the definition.
            – Sasha
            Nov 17 at 10:12










          • I followed the definition in Groupes de monodromie en géométrie algébrique. II. Lecture Notes in Mathematics, Vol. 340. Séminaire de Géométrie Algébrique du Bois-Marie 1967–1969 (SGA 7 II).
            – User X
            Nov 17 at 11:20












          • A precise reference should include expose and section numbers. In fact "axis" ("l'axe") in English is usually called "base locus". It surely doesn't follow from (ii). For instance, take $X subset mathbb{P}^2$ to be a smooth conic, and the pencil of lines through a point ON $X$.
            – Sasha
            Nov 17 at 14:44










          • Sorry I thought something wrong. Thanks.
            – User X
            Nov 17 at 19:07
















          Thanks for your answer. Btw can I ask another question (maybe I should start a new question but anyway) why we need the condition (i) in the definition? Isn’t it followed by (ii)?
          – User X
          Nov 17 at 9:39




          Thanks for your answer. Btw can I ask another question (maybe I should start a new question but anyway) why we need the condition (i) in the definition? Isn’t it followed by (ii)?
          – User X
          Nov 17 at 9:39












          Please, give a precise reference to the definition.
          – Sasha
          Nov 17 at 10:12




          Please, give a precise reference to the definition.
          – Sasha
          Nov 17 at 10:12












          I followed the definition in Groupes de monodromie en géométrie algébrique. II. Lecture Notes in Mathematics, Vol. 340. Séminaire de Géométrie Algébrique du Bois-Marie 1967–1969 (SGA 7 II).
          – User X
          Nov 17 at 11:20






          I followed the definition in Groupes de monodromie en géométrie algébrique. II. Lecture Notes in Mathematics, Vol. 340. Séminaire de Géométrie Algébrique du Bois-Marie 1967–1969 (SGA 7 II).
          – User X
          Nov 17 at 11:20














          A precise reference should include expose and section numbers. In fact "axis" ("l'axe") in English is usually called "base locus". It surely doesn't follow from (ii). For instance, take $X subset mathbb{P}^2$ to be a smooth conic, and the pencil of lines through a point ON $X$.
          – Sasha
          Nov 17 at 14:44




          A precise reference should include expose and section numbers. In fact "axis" ("l'axe") in English is usually called "base locus". It surely doesn't follow from (ii). For instance, take $X subset mathbb{P}^2$ to be a smooth conic, and the pencil of lines through a point ON $X$.
          – Sasha
          Nov 17 at 14:44












          Sorry I thought something wrong. Thanks.
          – User X
          Nov 17 at 19:07




          Sorry I thought something wrong. Thanks.
          – User X
          Nov 17 at 19:07


















           

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