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Assume $X\subset \mathbb{P}^n$ to be a smooth projective variety defined over an algebraic closed field $k$ (the characteristic of $k$ could be either $0$ or positive). For every point $P\in X$, denote by $T_P X$ its corresponding tangent space. And denote by $$ (T_P X)^{\vee}:=\{\text{hyperplanes } H\supset T_P X\} $$ the set of hyperplanes containing $T_P X$. In fact, if we consider the dual space $(\mathbb{P}^n)^{\vee}$, then $(T_P X)^{\vee}$ could be understood as a linear space in it. In particular, $\dim (T_P X)^{\vee}+\dim X=n-1$.

My question is, is it possible that there are infinitely many distinct points $P_1, P_2, \cdots$ such that $$ \bigcap_{i=1}^{\infty} (T_{P_i} X)^{\vee}\neq \emptyset? $$

So far, according to my best knowledge, when $X$ is a hypersurface, then the answer to the above question is negative. Since in this case, $(T_{P_i} X)^{\vee}$ is exact the Gauss image of $P_i$. And by a result of Zak, the Gauss map for a smooth variety is finite. But what will happen if $X$ is not a hypersurface? For instance, if $X$ is a complete intersection?

Edit: When I first posted the question, I forgot to exclude the obvious examples: the linear subspaces. So maybe I should ask what will happen when the degree of $X$ is strictly greater than $1$.

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    $\begingroup$ What if $X$ is a straight line? $\endgroup$
    – user178279
    Commented May 19, 2021 at 5:19
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    $\begingroup$ @virkkunen, Thanks. You are right, I should at least exclude this trivial case. I realize that my question is in fact related to the fiber of the conormal map. And your (counter) example is one of the example that the conormal map contract everything. $\endgroup$
    – Leo D
    Commented May 19, 2021 at 6:52

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You are asking whether there exists a hyperplane in $\mathbb{P}^n$ which is tangent to $X$ along a positive-dimensional subvariety. Consider the Veronese surface $V\subset \mathbb{P}^5$; by definition, the hyperplanes in $\mathbb{P}^5$ cut down the full linear system of conics in $\mathbb{P}^2$. In particular, there are hyperplanes which intersect $V$ along a double line, hence which are tangent to $V$ at each point of the line.

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  • $\begingroup$ Are there additional examples whenever the variety contains a linear subspace? Those are plentiful including for hypersurfaces. $\endgroup$ Commented May 19, 2021 at 5:23
  • $\begingroup$ I am not sure I understand the question : as the OP mentions, there are no such examples with hypersurfaces. All you need is a hyperplane section with a singular locus of dimension $>0$ —which can of course be a linear subspace. $\endgroup$
    – abx
    Commented May 19, 2021 at 6:06
  • $\begingroup$ @ZachTeitler if I didn't meantion that X is smooth, there you are right there are a lot of counter examples---those who contain linear components. But if I require smoothness, then maybe there is only one exceptional, the hyperplane. $\endgroup$
    – Leo D
    Commented May 19, 2021 at 6:54
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    $\begingroup$ Thanks @abx for your beautiful example. The idea of considering the Veronese makes everything so concrete. $\endgroup$
    – Leo D
    Commented May 19, 2021 at 6:59
  • $\begingroup$ I was thinking of a hyperplane tangent along a line in $X$. $\endgroup$ Commented May 19, 2021 at 11:53

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