Questions tagged [projective-geometry]

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5
votes
1answer
280 views

Golden ratio as a property of conic section (is it known?)

I am looking for a proof of a discovery as follows: Let $ABC$ be arbitrary triangle and $(\Omega)$ be an arbitrary circumconic of $ABC$ let $A'B'C'$ is its tangential triangle of $ABC$ respect to $(\...
1
vote
1answer
233 views

Thirteen-point conic and four-point line, are they new?

We know that Five points determine a conic and Two Points Determine a Line. Here I found a simple construct of a conic through $7$ points (in PS I note that how the conic through thirteen points) and ...
13
votes
1answer
1k views

Is it a new discovery on conic section?

I discovered a problem in plane geometry (there are some nice special cases) as follows: Let $ABC$ be a triangle and $\Omega$ be arbitrary circumconic. Let two points $A_b, A_c \in BC$, $B_c, B_a \in ...
3
votes
0answers
90 views

Nodes of rational plane sextic curves

According to the genus formula for plane curves, a nodal, irreducible rational plane curve $C\subset\mathbb{P}^2$ of degree six must have $10$ nodes. Now, if we take $10$ general points in $\mathbb{P}^...
3
votes
1answer
101 views

Projective invariants of the plane and cross ratio

I am looking for a reference for the following admittedly imprecise statement: Any projective invariant of n points in the projective plane may be expressed as a function of well-chosen cross-ratios. ...
9
votes
0answers
123 views

Projective planes over non-division rings

Is there a "right" notion of a projective plane over a general (unital, non-division) ring? Let me explain what type of object I am looking for. Let $R$ be an arbitrary (not necessarily ...
4
votes
0answers
113 views

Sections of fibrations of Kodaira dimension zero

Let $X$ be a projective variety with a morphism $f:X\rightarrow \mathbb{P}^1$, and let $F$ be a general fiber of $f$. Assume that $F$ in turn has a fibration $g_{F}:F\rightarrow S$ with rational ...
3
votes
0answers
84 views

Inverse flip of terminal 3-fold

Let $X$ be a terminal $3$-fold over $\mathbb{C}$ and $C\subset X$ a rational curve with normal bundle $N_{C/X} = \mathcal{O}_C(-2)\oplus\mathcal{O}_C(-k)$ for some integer $k > 0$. Assume that $-K_{...
2
votes
1answer
196 views

Tangent space to spaces of maps

Let $B = \{x_1,\dots,x_{d-2},y_1,\dots,y_k\}$ be a subscheme of $d-2+k$ distinct points of $\mathbb{P}^1$, and $g:B\rightarrow \mathbb{P}^2$ be a morphism mapping $x_1,\dots,x_{d-2}$ to a fixed point $...
1
vote
1answer
137 views

A question on linear projection of a smooth projective variety

Let $X$ be a smooth, projective $\mathbb{C}$-variety of dimension $n$. Fix a closed point $x \in X$ and an embedding of $X$ in $\mathbb{P}^m$ for some integer $m$. For a given $d$, denote by $\sigma_d ...
10
votes
1answer
383 views

Jumping conics in Grassmannians

Let $Gr(1,n)$ be the Grassmannian of lines in $\mathbb{P}^n$, and $f:\mathbb{P}^1\rightarrow Gr(1,n)$ a morphism of degree two. The pull-back $f^{*}S$ of the tautological bundle $S$ on $Gr(1,n)$ ...
0
votes
1answer
99 views

Moving general fibers of a fibration

Let $X$ be an irreducible projective variety over $\mathbb{C}$ admitting a morphism $\pi:X\rightarrow \mathbb{P}^1$ with connected fibers. We may assume that the general fiber of $\pi$ is smooth. My ...
2
votes
0answers
57 views

Geometrical meaning of a question from Marden

Let $T \in SL(2,\mathbb{C})$ be a normalised Möbius transformation. Then, $$|T(z) - T(w)| =|z-w||T'(z)^{\frac{1}{2}}||T'(w)^{\frac{1}{2}}|$$. The above is an exercise from Outer Circles by Marden (ex. ...
4
votes
1answer
155 views

Irreducibility of the base and of the general fiber

Let $f:X\rightarrow Y$ be a morphism of scheme over $\mathbb{C}$. Assume that $Y$ and the the general fiber $F_y = f^{-1}(y)$ of $f$ are irreducible. Does there exists an irreducible component $X'$ of ...
2
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0answers
81 views

Is the projective symmetry group of a polytope more general than its linear symmetry group?

Give a (convex) polytope $P\subset\Bbb R^d$ (the convex hull of finitely many points). Consider its linear and projective symmetry groups: \begin{align} \DeclareMathOperator{\Aut}{Aut} \...
4
votes
1answer
168 views

Moduli spaces and conic bundles

The moduli space $A_2(1,8)^{\operatorname{lev}}$ of $(1,8)$-polarized abelian surfaces with canonical level structure has a structure of conic bundle over $\mathbb{P}^2$ with a curve of degree $4$ as ...
2
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0answers
76 views

Canonical class & ring of projective space $\mathbb{P}^n$ in differential geometry

David Mumford remarks in his book Algebraic Geometry I, Complex Projective Varieties on page 109 that the fact that the canonical ring $\oplus_{k=0}^{\infty} \Omega_{k, \mathbb{P}^n}$ of projective ...
3
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1answer
130 views

Degenerations of hyperelliptic coverings

Take six distinct points $p_1,\dots,p_6\in\mathbb{P}^1$ and consider the double covering $f:C\rightarrow \mathbb{P}^1$ ramified over $p_1,\dots,p_6\in\mathbb{P}^1$. Then $C$ is a smooth curve of genus ...
2
votes
1answer
58 views

Smallest subset in $P^2 \mathbf F_q$ which cannot be disjointed from itself by a homography

Let $q$ be a power of a prime and $S \subseteq \mathrm P^2 \mathbf F^q$ such that $$ \forall g \in \operatorname{PGL}(3,q), gS \cap S \neq \emptyset.$$ Can it be that $\vert S \vert < 1+q$ ? (I ...
3
votes
0answers
71 views

Determinantal representation of joins

Let $X^n = Z(I_X)\subset\mathbb{P}^N$ be an $n$-dimensional irreducible and non degenerate variety. Consider a linear subspace $H = Z(I_H)\subset\mathbb{P}^N$ of dimension $N-n-2$ disjoint from $X$. ...
10
votes
1answer
180 views

Set theoretic equation for Veronese varieties

Consider the embedding $f:\mathbb{P}^n\rightarrow\mathbb{P}^N$ induced by the complete linear system of degree $d$ hypersurfaces of $\mathbb{P}^n$. Its image $V_{n,\,d}$ is degree $d$ Veronese variety ...
3
votes
1answer
102 views

3-secant lines of a projective curve

Consider a smooth projective curve $C\subset\mathbb{P}^n$. Let $G(1,n)$ the Grassmannian of lines of $\mathbb{P}^n$. The variety $S_2(C)\subset G(1,n)$ parametrizing lines that are secant to $C$ (i.e.,...
5
votes
1answer
272 views

Higher order inflection points

Consider a smooth plane curve $X\subset\mathbb{P}^2$ of degree $d$. We will say that $x\in X$ is an inflection point of order $s$ if the tangent line $T_xX$, of $X$ at $x\in X$, intersects $X$ in $x\...
3
votes
1answer
143 views

Configuration of points on a plane curve

Let $C\subset\mathbb{P}^2$ be a smooth plane curve of degree six. On $C$ there are $21$ points given as the intersection points of two lines choosen among a set of seven lines. More precisely there ...
2
votes
1answer
134 views

Picard groups of determinantal varieties

Consider a general $4\times 4$ matrix: $$ X:=\left( \begin{array}{cccc} X_0 & X_1 & X_2 & X_3 \\ X_4 & X_5 & X_6 & X_7 \\ X_8 & X_9 & X_{10} & X_{11} \\ X_{12} &...
2
votes
1answer
202 views

Help about “Varieties with small Dual Varieties” by L.Ein

I'm studying the paper "Varieties with small Dual Varieties" by L.Ein and in the construction he gives about the $10-$dimensional spinor variety $S_4 \subset \mathbb{P}^{15}$ I'm finding ...
4
votes
1answer
153 views

Linear spaces secant to Veronese varieties

The following question makes sense in a more general setting but for sake of simplicity let me stick to a particular case. Consider the degree three Veronese embedding $V\subset\mathbb{P}^9$ of $\...
5
votes
1answer
184 views

Software computing dimension and degree

Assume a projective scheme $X_{k_1,\dots,k_r}\subset\mathbb{P}^n$ is given as the set of common solutions of homogeneous polynomials $F_1(x_0,\dots,x_n),\dots,F_s(x_0,\dots,x_n)$, where the $F_i$ ...
1
vote
0answers
67 views

Strict transforms of higher codimension subvarieties

The following question could be posed in a more general context but for simplicity I will stick to a particular case. Let $X\subset\mathbb{P}^9$ be a smooth variety of degree $d$ and dimension $6$. ...
2
votes
0answers
131 views

Quadrics tangent to lines

I think that the following must be a basic question in enumerative geometry. Take a line $L\subset\mathbb{P}^3$. The quadric surfaces in $\mathbb{P}^3$ that are tangent to $L$ are parametrized by a ...
0
votes
1answer
53 views

Vertices of 2 self-polar triangles lie on conic

I have conic $\gamma$ and two self-polar triangles $ABC$, $XYZ$ with respect to my conic. Why can I construct a one conic through $ABCXYZ$?
9
votes
2answers
572 views

Picard group of a cubic hypersurface

Consider the following cubic hypersurface in $\mathbb{P}^5$: $$ X = \{z_0z_3z_5-z_1^2z_5-z_0z_4^2+2z_1z_2z_4-z_2^2z_3 = 0\}\subset\mathbb{P}^5 $$ The singular locus of $X$ is the Veronese surface $V\...
4
votes
0answers
71 views

Maximal abelian subgroups of the full collineation group $\mathrm{P\Gamma L}_3(q)$

Is there a convenient list of the maximal abelian subgroups of the projective semilinear group $\mathrm{P\Gamma L}_3(K) \cong \mathrm{PGL}_3(K) \rtimes \mathrm{Gal}(K)$ for $K$ a finite field? This is ...
2
votes
0answers
89 views

Divisorial contraction to a non-normal variety

Consider a divisorial contraction $f:X\rightarrow Y$, between projective varieties, contracting an irreducible divisor $D\subset X$ to a subvariety $Z\subset Y$ of codimension at least two, and which ...
1
vote
0answers
94 views

Trisecant lines to curves in $\mathbb{P}^3$: a reference request

I'm starting to study the geometry of the locus of trisecant lines to a space curve $C \subset \mathbb{P}^3$. The main references, cited in almost all the papers, regarding the specific calculations ...
5
votes
2answers
155 views

Surface of type $(2,2)$ on the Segre cubic scroll $\mathbb{P}^1 \times \mathbb{P}^2 \subset \mathbb{P}^5$

Let $S=\mathbb{P}^1 \times \mathbb{P}^2 \subset \mathbb{P}^5$ embedded with the Segre embedding given by $\mathcal{O}_S(1,1)$. If we intersect $S$ with a general smooth quadric $Q \subset \mathbb{P}^5$...
1
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0answers
124 views

Cubic surface in $\mathbb{P}^3$ singular along a line

Maybe it is a stupid question but I'm not able to find the answer anywhere else. My goal is to prove in an "algebraic geometry fashion" that $\sqrt{n}$ is not a rational number for $n$ not a ...
0
votes
0answers
79 views

Lines through the origin every pair of which meet at the same angle

This item isn't getting attention, so I'll try it here: begin quote The three lines through antipodal pairs of centers of faces of a cube meet each other pairwise at $90^\circ$ angles. The three lines ...
4
votes
1answer
139 views

Smoothness of moduli spaces of stable maps

If $X$ is a projective variety the moduli space of stable maps $\overline{M}_{0,0}(X,\beta)$ is a normal variety with finite quotient singularities. Can the pairs $(X,\beta)$ such that $\overline{M}_{...
1
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0answers
105 views

Kähler fiber space with base and fiber projective

Let $X$ be a Kähler manifold, $Y$ be a projective manifold, if $X$ exits a smooth fibration over $Y$ such that all the fibers are projective manifolds, then is $X$ a projective mannifold? If we do not ...
0
votes
1answer
88 views

Does projective transformation preserve convexity? [closed]

Does projective transformation preserve convexity? Notice: Ignore the trivial case which projects a convex curve to a straight line.
5
votes
1answer
213 views

Isomorphisms of complete intersections

Let $X, Y\in \mathbb{P}^n$ be two singular Fano complete intersections of the same multidegree $(d_1,…,d_r)$. If we assume there is an isomorphism $f\colon X\rightarrow Y$ are there any assumptions so ...
3
votes
0answers
64 views

Infinite-dimensional quasifields

In their seminal paper on translation planes (The Construction of Translation Planes from Projective Spaces, Journal of Algebra 1:85-102, 1964, https://doi.org/10.1016/0021-8693(64)90010-9), Bruck and ...
5
votes
0answers
85 views

Divisorial contractions and singularities

I have a smooth $6$-fold $X\subset\mathbb{P}^n$ and a divisor $D\subset X$ cut out by a quadratic polynomial. I know that $D$ in singular along a smooth $3$-fold $Y\subset X$, and that if $Z$ is the ...
2
votes
1answer
240 views

A question on a Macaulay2 computation

I have an ideal $I$ generated by quadratic and cubic homogeneous polynomials in $10$ variables. Macaulay2 tells me that $I$ defines an irreducible variety $X$ of dimension $5$ and degree $10$ in $\...
9
votes
1answer
504 views

What is the automorphism group of the projective line minus $n$ points?

$\DeclareMathOperator{\AGL}{\operatorname{AGL}}\DeclareMathOperator{\PGL}{\operatorname{PGL}}$What is the automorphism group of $\mathbb P^1$ minus $n$ points (let's say over an algebraically closed ...
1
vote
0answers
158 views

Embeddings of Hirzebruch surfaces $\mathbb{P}(\mathcal{O}_{\mathbb{P}^1}\oplus\mathcal{O}_{\mathbb{P}^1}(n))$

Let $X_n=\mathbb{P}(\mathcal{O}_{\mathbb{P}^1}\oplus\mathcal{O}_{\mathbb{P}^1}(n))$ be the $n-$th Hirzebruch surface. We know that for $d>0$ and higher $k>>0$ the linear system $$\mathcal{L}_{...
7
votes
1answer
209 views

Strict transform of a tangent curve under blow-up

$\DeclareMathOperator{\Bl}{\operatorname{Bl}}$It is known that if we have a projective variety $X$ and a projective smooth subvariety $Y$ then the exceptional divisor $E \subset \Bl_{Y}X$ of the blow-...
4
votes
0answers
127 views

Example of a computation of the volume of a subvariety in projective space $\mathbb{P}^n$

Let us consider the projective space $\mathbb{P}^n$ with the standard Fubini Study metric. I searched all over the internet but I can't find an example of a calculation of the volume for a projective ...
5
votes
0answers
382 views

When a fibration over a projective manifold is projective?

Let $X$ be a complex manifold, $B$ be a complex projective manifold, consider a smooth fibration $\pi:X\rightarrow B$ such that all the fibers of $\pi$ are simply-connected projective manifolds, then ...

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