arXiv subjects
Publications and source records attributed to Jun Bo Lau.
In this work, we prove that the permutation group of a Reed-Solomon code is given by the polynomials of degree one that leave the set of evaluation points invariant. Our results provide a straightforward proof of the well-known cases of the permutation group of the Reed-Solomon code when the set of evaluation points is the whole finite field or the multiplicative group.
We introduce a ``non-orientable'' variation of Serre's definition of a graph, which we call an abstract isogeny graph. These objects capture the combinatorics of the graphs $G(p,\ell,H)$, the $\ell$-isogeny graphs of supersingular elliptic curves with $H$-level structure. In particular they allow for the study of non-backtracking walks, primes, and zeta functions. We prove an analogue of Ihara's determinant formula for the zeta function of an abstract isogeny graph. For $B_1(N) \subseteq H \subseteq B_0(N)$ and $p > 3$, we use this formula to relate the Ihara zeta function of $G(p,\ell,H)$ to the Hasse-Weil zeta functions of the modular curves $X_{H, {\mathbb{F}_{\ell}}}$ and $X_{H \times B_0(p), \mathbb{F}_{\ell}}$. As applications, we give an explicit formula relating point counts on $X_0(pN)_{\mathbb{F}_{\ell}}$ and $X_0(N)_{\mathbb{F}_{\ell}}$ to cycle counts in $G(p,\ell,B_0(N))$ and prove that the number of non-backtracking cycles of length $r$ in $G(p,\ell,B_0(N))$ is asymptotic to $\ell^r$.
For each open subgroup $H\leq \operatorname{GL}_2(\widehat{\mathbb{Z}})$, there is a modular curve $X_H$, defined as a quotient of the full modular curve $X(N)$, where $N$ is the level of $H$. The genus formula of a modular curve is well known for $X_0(N)$, $X_1(N)$, $X(N)$, $X_{\mathrm{sp}}(N)$, $X_{\mathrm{ns}}(N)$, and $X_{S_4}(p)$ for $p$ prime. We explicitly work out the invariants of the genus formulas for $X_{\mathrm{sp}}^+(N)$, $X_{\mathrm{ns}}^+(N)$, and $X_{\text{arith},1}(M,MN)$. In Table $1$, we provide the invariants of the genus formulas for all of the modular curves listed.
We compile a complete list of isomorphism class representatives of curves of genus 6 over $\mathbb{F}_2$. We use explicit descriptions of canonical curves in each stratum of the Brill--Noether stratification of the moduli space $\mathcal{M}_6$, due to Mukai in the generic case. Our computed value of $\#\mathcal{M}_6(\mathbb{F}_2)$ agrees with the Lefschetz trace formula as recently computed by Bergstrom--Canning--Petersen--Schmitt.
Coleman integrals is a major tool in the explicit arithmetic of algebraic varieties, notably in the study of rational points on curves. One of the inputs to compute Coleman integrals is the availability of an affine model. We develop a model-free algorithm that computes single Coleman integrals between any two points on modular curves. Using Hecke operators, any Coleman integral can be broken down into a sum of tiny integrals. We illustrate this using several examples computed in SageMath and Magma. We also suggest some future directions for this work, including a possible extension to iterated Coleman integrals.