arXiv subjects
Publications and source records attributed to Youngsu Kim.
We provide a simplified proof of the following special case of Wegner's conjecture: every planar graph of maximum degree at most three admits a distance-2 coloring with at most eight colors. Our main contribution is significant simplification of the most technically challenging part of Wegner's proof: the case involving the removal of a 5-cycle.
The MultiplicitySequence package for Macaulay2 computes the multiplicity sequence of a graded ideal in a standard graded ring over a field, as well as several invariants of monomial ideals related to integral dependence. We discuss two strategies implemented for computing multiplicity sequences: one via the bivariate Hilbert polynomial, and the other via the technique of general elements.
The Picard group of an undirected graph is a finitely generated abelian group, and the Jacobian is the torsion subgroup of the Picard group. These groups can be computed by using the Smith normal form of the Laplacian matrix of the graph or by using chip-firing games associated with the graph. One may consider its generalization to directed graphs based on the Laplacian matrix. We compute Picard groups and Jacobians for several classes of directed trees, cycles, wheel, and multipartite graphs.
We show that the log canonical threshold of a generic determinantal variety and its generic link are the same.
Consider the rational map $ϕ: \mathbb{P}^{n-1}_{\mathbf k} \stackrel{[f_0:\cdots: f_n]}{\longrightarrow} \mathbb{P}^{n}_{\mathbf k}$ defined by homogeneous polynomials $f_0,\dots,f_n$ of the same degree $d$ in a polynomial ring $R=\mathbf k [x_1,\dots,x_n]$ over a field $\mathbf k$. Suppose $I=(f_0,\dots,f_n)$ is a height two perfect ideal satisfying $μ(I_p)\leq\dim R_p$ for $p\in \operatorname{Spec} (R) \setminus V(x_1,\dots, x_n)$. We study the equations defining the graph of $ϕ$ whose coordinate ring is the Rees algebra $R[It]$. We provide new methods to construct these equations using work of Buchsbaum and Eisenbud. Furthermore, for certain classes of ideals satisfying the conditions above, our methods lead to explicit equations defining Rees algebras of the ideals in these classes. These classes of examples are interesting, in that, there are no known methods to compute the defining ideal of the Rees algebra of such ideals. These new methods also give rise to effective criteria to check that $ϕ$ is birational onto its image.
In this note, we study the Cohen-Macaulayness of non-Noetherian rings. We show that Hochster's celebrated theorem that a finitely generated normal semigroup ring is Cohen-Macaulay does not extend to non-Noetherian rings. We also show that for any valuation domain $V$ of finite Krull dimension, $V[x]$ is Cohen-Macaulay in the sense of Hamilton-Marley.
Let $R$ be a Noetherian local ring and $I$ an $R$-ideal. It is well-known that if the associated graded ring $\gr_I(R)$ is Cohen-Macaulay (Gorenstein), then so is $R$, but the converse is not true in general. In this paper we investigate the Cohen-Macaulayness and Gorensteinness of the associated graded ring $\gr_I(R)$ under the hypothesis of the extended Rees algebra $R[It,t^{-1}]$ is quasi-Gorenstein or the associated graded ring $\gr_I(R)$ is a domain.
We show that if a graded submodule of a Noetherian module cannot be written as a proper intersection of graded submodules, then it cannot be written as a proper intersection of submodules at all. More generally, we show that a natural extension of the index of reducibility to the graded setting coincides with the ordinary index of reducibility. We also investigate the question of uniqueness of the components in a graded-irreducible decomposition, as well as the relation between the index of reducibility of a non-graded ideal and that of its largest graded subideal.
Let $I$ be a regular proper ideal in a Noetherian ring $R$, let $e \ge 2$ be an integer, let $\mathbf T_e = R[u,tI,u^{\frac{1}{e}}]' \cap R[u^{\frac{1}{e}},t^{\frac{1}{e}}]$ (where $t$ is an indeterminate and $u =\frac{1}{t}$), and let $\mathbf r_e = u^{\frac{1}{e}} \mathbf T_e$. Then the Itoh (e)-valuation rings of $I$ are the rings $(\mathbf T_e/z)_{(p/z)}$, where $p$ varies over the (height one) associated prime ideals of $\mathbf r_e$ and $z$ is the (unique) minimal prime ideal in $\mathbf T_e$ that is contained in $p$. We show, among other things: (1) $\mathbf r_e$ is a radical ideal if and only if $e$ is a common multiple of the Rees integers of $I$. (2) For each integer $k \ge 2$, there is a one-to-one correspondence between the Itoh (k)-valuation rings $(V^*,N^*)$ of $I$ and the Rees valuation rings $(W,Q)$ of $uR[u,tI]$; namely, if $F(u)$ is the quotient field of $W$, then $V^*$ is the integral closure of $W$ in $F(u^{\frac{1}{k}})$. (3) For each integer $k \ge 2$, if $(V^*,N^*)$ and $(W,Q)$ are corresponding valuation rings, as in (2), then $V^*$ is a finite integral extension domain of $W$, and $W$ and $V^*$ satisfy the Fundamental Equality with no splitting. Also, if $uW = Q^e$, and if the greatest common divisor of $e$ and $k$ is $d$, and $c$ is the integer such that $cd = k$, then $QV^* = {N^*}^c$ and $[(V^*/N^*):(W/Q)] = d$. Further, if $uW = Q^e$ and $k = qe$ is a multiple of $e$, then there exists a unit $θ_{e} \in V^*$ such that $V^* = W[θ_{e},u^{\frac{1}{k}}]$ is a finite free integral extension domain of $W, QV^* = {N^*}^q, N^* = u^{\frac{1}{k}}V^*$, and $[V^*:W] = k$. (4) If the Rees integers of $I$ are all equal to $e$, then $V^* = W[θ_e]$ is a simple free integral extension domain of $W, QV^* = N^* = u^{\frac{1}{e}}V^*$, and $[V^*:W] = e = [(V^*/N^*):(W/Q)]$.
Let I be a finitely supported complete m-primary ideal of a regular local ring (R, m). We consider singularities of the normalization of the blow-up Proj R[It] of I. A theorem of Lipman implies that the ideal I has a unique factorization as a star-product of special star-simple complete ideals with possibly negative exponents for some of the factors. If the normalization of the projective model Proj R[It] is regular, we prove that it is the regular model obtained by blowing up the finite set of base points of I. Extending work of Lipman and Huneke-Sally in dimension 2, we prove that every local ring S on the normalization of Proj R[It] that is a unique factorization domain is regular. Moreover, if dim S is at least 2 and S dominates R, then S is an infinitely near point to R, that is, S is obtained from R by a finite sequence of local quadratic transforms.
We consider the following question concerning the equality of ordinary and symbolic powers of ideals. In a regular local ring, if the ordinary and symbolic powers of a one-dimensional prime ideal are the same up to its height, then are they the same for all powers? We provide supporting evidence of a positive answer for classes of prime ideals defining monomial curves or rings of low multiplicities.