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The classical finite field Kakeya conjecture state as following (for conveinent, all version of kakeya conjecture is state in hasdorff dimension version):
$\mathrm{Finite\ Field\ Kakeya\ Conjecture}$: Let $\mathrm{F}=\mathrm{F}/\mathrm{qF}$ be a finite field, let $K \subseteq \mathrm{F}^{\mathrm{n}}$ be a Kakeya set, i.e. for each vector $y \in \mathbf{F}^{n}$ there exists $x \in \mathbf{F}^{n}$ such that $K$ contains a line $\{x+t y: t \in \mathbf{F}\}$. Then the set $K$ has size at least $c_{n}q^{n}$ where $c_{n}>0$ is a constant that only depends on $n$.

.....................(*)

Dvir's proof of the finite field version of Kakeya conjecture is elegant and influential. On the other hand, it seems the corresponding version of (*) in $\mathrm{R^n}$ is that,

a set $K$ if satisfied, for every $\theta \in S^{n-1}$ $K$ contain a line $l_{\theta, a_{\theta}}$ and $l$ has direction $\theta$ , then the haussdorff dimension of $K$ is $n$.

But this version in Euclidean space is not essential difficult, in particuler because every line in $R^n$ is not compact and a suitable infinite sequence of line shall group a set which has hausdroff dimension $n$. The original version of Kakeya conjecture in $R^n$ change line to a segement with length 1. So, is there a possibility that a more subtle version of finite field analoge of Kakeya conjecture maybe true? i.e.:

$\mathrm{Version \ 1}$: Fix a sequence $\{a_m\}_{m=1}^{\infty}$, $a_{m} \to \infty$ as $m \to \infty$ and we define a set $K$ in $F_{p_m}^n= (Z/Z_{p_m})^n $ to be a Kakeya set iff for every vector $v\in F_{p_m}^n, v\neq 0 $, such that $K$ contain consecutively(or not) at least $a_n$ point in a line $\{x+t v: t \in \mathbf{F}\}$ for some $x\in F_{p_m}^n$, i.e. contain every direction line with at least $a_m$ elements, then exist $c>0$ only depend on $m$ such that $|K|\geq cp_m^{n-1}$?\

or more easier problem(compare with version 1) but the directly use of polynominal method Invalidation is a following:

$\mathrm{Version \ 2}$: we define a set $K$ in $F_{p_m}^n= (Z/Z_{p_m})^n $ to be a Kakeya set iff for every vector $v\in F_{p_m}^n, v\neq 0 $, such that $K$ contain consecutive(or not) at least $p_m^{\frac{1}{2}}$ point in a line $\{x+t v: t \in \mathbf{F}\}$ for some $x\in F_{p_m}^n$, i.e. contain every direction line with at least $p_m^{\frac{1}{2}}$ elements, then exist $c>0$ only depend on $n$ such that $|K|\geq cp_m^{n-1}$?

for $\mathrm{Version \ 2}$, the polynomial argument gain a bound $O(p^{\frac{m}{2}})$ which coincide with the combinatorics argument two point depending a line. And it is esay to show if take $a_m=2, \forall m \in N^*$ in $\mathrm{Version \ 1}$, then this set $K $ in general do not have full dimension, this corresponde to the situation in $R^n$ when we have a scale $\delta$ kakeya set but instead of $T^{\delta}_{a_{\theta},\theta}$ is a $1\times \delta^{n-1}$ tube, we only have a $O(\delta) \times\delta^{n-1}$ tube.

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In his paper "On the size of Kakeya sets in finite fields" (where the proof of the finite field Kakeya conjecture has appeared), Dvir also introduces the notion of a $(\delta,\gamma)$-Kakeya set, which is, essentially, a set $K\subset\mathbb F_q^n$ with the following property: there are at least $\delta(q^n-1)/(q-1)$ directions in $\mathbb F_q^n$ such that in each of these directions there is a line containing at least $\gamma q$ points of $K$. He then proves the following result:

Theorem. If $K\subset\mathbb F_q^n$ is a $(\gamma,\delta)$-Kakeya set, then $$ |K| \ge \binom{n+d-1}{n-1}, $$ where $d=\lfloor q\cdot\min\{\delta,\gamma\}\rfloor-2$.

Applying this theorem with $\delta=1$ and $\gamma=\varepsilon q^{-1+c}$, we conclude that if $K\subset\mathbb F_q^n$ contains at least $\varepsilon q^c$ collinear points in every direction, then $$ |K| \ge \binom{n+d-1}{n-1},\quad d=\lfloor \varepsilon q^c\rfloor-2. $$

Does this answer the question?

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  • $\begingroup$ thank you for the answer, the point is if change $\gamma q$ to $\gamma q^c, 0<c<1$, is the kakeya conjecture still true, becasue if a line in $(Z/pZ)^n$ is too long, it is unreasonable the analoge in $R^n$ is a segement with length 1 $\endgroup$
    – katago
    Commented Nov 16, 2020 at 14:39
  • $\begingroup$ @xyh: Dvir's result seems to answer this; I inserted an explanation into my answer. $\endgroup$
    – Seva
    Commented Nov 16, 2020 at 15:34
  • $\begingroup$ You are totally right, and in general in the article using polynomial method we can only get a bound of $(1,q^{-(1-c)})$-kakeya set $K$ has lower bound $|K|=O(q^{cn})$, which coincide with the obersevation in the last of my question. And in general wheather we have $(1,q^{-(1-c)})$-kakeya set also has full dimension or there is a counterexample? This is not discuss in the original article of Divr. $\endgroup$
    – katago
    Commented Nov 16, 2020 at 16:27
  • $\begingroup$ Sorry, I am fool, there exist easy example to explain the result of Divr is optimal. $\endgroup$
    – katago
    Commented Nov 16, 2020 at 17:37
  • $\begingroup$ The counterexample I construct is wrong, I still can not figure out weather there exist a $(1,q^{−(1−c)})$-kakeya set $K$ such that $|K|=O(q^{(1-\epsilon)n})$($\epsilon>0$ relay on $c$) and K lie in a suffice large $(Z/qZ)^n, i.e. q\to \infty$ or not. $\endgroup$
    – katago
    Commented Nov 17, 2020 at 6:15

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