On a class of power ideals

In this paper we study the class of power ideals generated by the $k^n$ forms $(x_0+\xi^{g_1}x_1+\ldots+\xi^{g_n}x_n)^{(k-1)d}$ where $\xi$ is a fixed primitive $k^{th}$-root of unity and $0\leq g_j\leq k-1$ for all $j$. For $k=2$, by using a $\mathbb{Z}_k^{n+1}$-grading on $\mathbb{C}[x_0,\ldots,x_n]$, we compute the Hilbert series of the associated quotient rings via a simple numerical algorithm. We also conjecture the extension for $k>2$. Via Macaulay duality, those power ideals are related to schemes of fat points with support on the $k^n$ points $[1:\xi^{g_1}:\ldots:\xi^{g_n}]$ in $\mathbb{P}^n$. We compute Hilbert series, Betti numbers and Gr\"obner basis for such $0$-dimensional schemes. This explicitly determines the Hilbert series of the power ideal for all $k$: that this agrees with our conjecture for $k>2$ is supported by several computer experiments

On some invariants in numerical semigroups and estimations of the order bound

We study suitable parameters and relations in a numerical semigroup S. When S is the Weierstrass semigroup at a rational point P of a projective curve C, we evaluate the Feng-Rao order bound of the associated family of Goppa codes. Further we conjecture that the order bound is always greater than a fixed value easily deduced from the parameters of the semigroup: we also prove this inequality in several cases

An application of type sequences to the blowing-up

Let I be an m-primary ideal of a one-dimensional, analytically irreducible and residually rational local Noetherian domain R. Given the blowing-up of R along I we establish connections between the type-sequence of R and classical invariants like multiplicity, genus and reduction exponent of I.Comment: 19 page

On semigroup rings with decreasing Hilbert function

In this paper we study the Hilbert function HR of one-dimensional semigroup rings R = k[[S]]. For some classes of semigroups, by means of the notion of support of the elements in S, we give conditions on the generators of S in order to have decreasing HR. When the embedding dimension v and the multiplicity e verify v + 3 ? e ? v + 4, the decrease of HR gives explicit description of the Apery set of S. In particular for e = v+3, we classify the semigroups with e = 13 and HR decreasing, further we show that HR is non-decreasing if e < 12. Finally we deduce that HR is non-decreasing for every Gorenstein semigroup ring with e ? v + 4

On the Hilbert function of general fat points in P1×P1\mathbb{P}^1 \times \mathbb{P}^1

We study the bi-graded Hilbert function of ideals of general fat points with same multiplicity in $\mathbb{P}^1\times\mathbb{P}^1$. Our first tool is the multiprojective-affine-projective method introduced by the second author in previous works with A.V. Geramita and A. Gimigliano where they solved the case of double points. In this way, we compute the Hilbert function when the smallest entry of the bi-degree is at most the multiplicity of the points. Our second tool is the differential Horace method introduced by J. Alexander and A. Hirschowitz to study the Hilbert function of sets of fat points in standard projective spaces. In this way, we compute the entire bi-graded Hilbert function in the case of triple points.Comment: 25 pages; minor changes (Remark 1.7 added and Example 3.13 improved