arXiv subjects

Semin Yoo

Publications and source records attributed to Semin Yoo.

24 records · Page 2Linked to original sources

arXiv2021

An Isometric Invariant of Quadratic Spaces over Finite Fields

Let $\mathbb{F}_{q}$ be the finite field with an odd prime power $q$. In this paper, we construct a new isometric invariant of combinatorial type on $(\mathbb{F}^{n}_{q},\text{dot}_{n})$, where $\text{dot}_{n}(\mathbf{x}):=x_{1}^{2}+\cdots+x_{n}^{2}$. Additionally, using counts from our new invariant, we give a new proof of Minkowski's formula on the size of spheres over finite fields. We also show which types of quadratic subspaces can be embedded in $(\mathbb{F}_{q}^{n},\text{dot}_{n})$.

math.CO↗

arXiv2021

Continuously Increasing Subsequences of Random Multiset Permutations

For a word $π$ and integer $i$, we define $L^i(π)$ to be the length of the longest subsequence of the form $i(i+1)\cdots j$, and we let $L(π):=\max_i L^i(π)$. In this paper we estimate the expected values of $L^1(π)$ and $L(π)$ when $π$ is chosen uniformly at random from all words which use each of the first $n$ integers exactly $m$ times. We show that $\mathbb{E}[L^1(π)]\sim m$ if $n$ is sufficiently larger in terms of $m$ as $m$ tends towards infinity, confirming a conjecture of Diaconis, Graham, He, and Spiro. We also show that $\mathbb{E}[L(π)]$ is asymptotic to the inverse gamma function $Γ^{-1}(n)$ if $n$ is sufficiently large in terms of $m$ as $m$ tends towards infinity.

math.CO↗

arXiv2021

A combinatorial correspondence between finite Euclidean geometries and symmetric subsets of $\mathbb{Z}/n\mathbb{Z}$

$q$-analogues of quantities in mathematics involve perturbations of classical quantities using the parameter $q$, and revert to the original quantities when $q$ goes $1$. An important example is the $q$-analogues of binomial coefficients which give the number of $k$-dimensional subspaces in $\mathbb{F}_{q}^{n}$. When $q$ goes to $1$, this reverts to the binomial coefficients which measure the number of $k$-sets in $\left [ n \right ]$. Dot-analogues of $q$-binomial coefficients were studied by Yoo (2019) in order to investigate combinatorics of quadratic spaces over finite fields. The number of $k$-dimensional quadratic spaces of $(\mathbb{F}_{q}^{n},x_{1}^{2}+x_{2}^{2}+\cdots+x_{n}^{2})$ which are isometrically isomorphic to $(\mathbb{F}_{q}^{k},x_{1}^{2}+x_{2}^{2}+\cdots+x_{k}^{2})$ can be also described as analogous to binomial coefficients, called the dot-binomial coefficients, $\binom{n}{k}_{d}$. In this paper, we study a combinatorial correspondence between this finite Euclidean geometries and symmetric subsets of $\mathbb{Z}/n\mathbb{Z}$. In addition, we show that dot-binomial coefficients can be expressed in terms of $q$-binomial coefficients and polynomials, and we prove that dot-binomial coefficients are polynomials in $q$. Furthermore, we study the properties of the polynomials given by the dot binomial coefficients $\binom{n}{k}_{d}$.

math.CO↗

arXiv2020

Embeddings of quadratic spaces over the field of $p$-adic numbers

Nondegenerate quadratic forms over $p$-adic fields are classified by their dimension, discriminant, and Hasse invariant. This paper uses these three invariants, elementary facts about $p$-adic fields and the theory of quadratic forms to determine which types of quadratic spaces -- including degenerate cases -- can be embedded in the Euclidean $p$-adic space $(\mathbb{Q}_{p}^{n},x_{1}^{2}+\cdots+x_{n}^{2})$, and the Lorentzian space $(\mathbb{Q}_{p}^{n},x_{1}^{2}+\cdots+x_{n-1}^{2}+λx_{n}^{2})$, where $\mathbb{Q}_{p}$ is the field of $p$-adic numbers, and $λ$ is a nonsquare in the finite field $\mathbb{F}_{p}$. Furthermore, the minimum dimension $n$ that admits such an embedding is determined.

math.CO↗

arXiv2020

Incidences between quadratic subspaces over finite fields

Let $\mathbb{F}_{q}$ be a finite field of order $q$, where $q$ is an odd prime power. A quadratic subspace $(W,Q)$ of $(\mathbb{F}_{q}^{n},x_{1}^{2}+x_{2}^{2}+\cdots+x_{n}^{2})$ is called dot$_{k}$-subspace if $Q$ is isometrically isomorphic to $x_{1}^{2}+x_{2}^{2}+\cdots+x_{k}^{2}$. In this paper, we obtain bounds for the number of incidences $I(\mathcal{K},\mathcal{H})$ between a collection $\mathcal{K}$ of dot$_{k}$-subspaces and a collection $\mathcal{H}$ of dot$_{h}$-subspaces when $h \geq 4k-4$, which is given by \[\left | I(\mathcal{K},\mathcal{H})-\frac{|\mathcal{K}||\mathcal{H}|}{q^{k(n-h)}}\right | \lesssim q^{\frac{k(2h-n-2k+4)+h(n-h-1)-2}{2}}\sqrt{|\mathcal{K}||\mathcal{H}|}. \] In particular, we improve the error term obtained by Phuong, Thang and Vinh (2019) for general collections of affine subspaces in the presence of our additional conditions.

math.CO↗

arXiv2020

Graphs associated with orthogonal collections of k-planes over finite fields

We study graphs coming from quadratic spaces over finite fields via orthogonality which generalize a recent result given by Bishnoi, Ihringer, and Pepe (2019). More precisely, we study the graph $Γ^{\square}(n,k,q)$ as follows: the vertex set is the set of $k$-dimensional quadratic subspaces of a fixed Lorentzian quadratic space $(\mathbb{F}_{q}^{n},x_{1}^{2}+\cdots+x_{n-1}^{2}+λx_{n}^{2})$ which are isometrically isomorphic to $x_{1}^{2}+\cdots+x_{k}^{2}$. Here $λ$ is a nonsquare in $\mathbb{F}_{q}$, and two vertices $x,y$ are adjacent if $x \subseteq y^{\perp}$.

math.CO↗