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.
algebraic-geometry complex-geometry intersection-theory
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up vote
2
down vote
favorite
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
add a comment |
up vote
2
down vote
favorite
up vote
2
down vote
favorite
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
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
algebraic-geometry complex-geometry intersection-theory
edited Nov 16 at 22:30
asked Nov 16 at 21:41
User X
17411
17411
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1 Answer
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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 $mathbb{P}^{n*}$.
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
add a comment |
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
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 $mathbb{P}^{n*}$.
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
add a comment |
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 $mathbb{P}^{n*}$.
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
add a comment |
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 $mathbb{P}^{n*}$.
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 $mathbb{P}^{n*}$.
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
add a comment |
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
add a comment |
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