Derivation module and the Hilbert-Kunz multiplicity of the co-ordinate ring of a projective monomial curve

Abstract

Let $n_0, n_1, \ldots, n_p$ be a sequence of positive integers such that $n_0 < n_1 < \cdots < n_p$ and $\mathrm{gcd}(n_0,n_1, \ldots,n_p) = 1$. Let $S = \langle (0,n_p), (n_0,n_p-n_0),\ldots,(n_{p-1},n_p-n_{p-1}), (n_p,0) \rangle$ be an affine semigroup in $\mathbb{N}^2$. The semigroup ring $k[S]$ is the co-ordinate ring of the projective monomial curve in the projective space $\mathbb{P}_k^{p+1}$, which is defined parametrically by \begin{center} $x_0 = v^{n_p}, \quad x_1 = u^{n_0}v^{n_p-n_0},\quad \ldots , \quad x_p= u^{n_{p-1}}v^{n_p-n_{p-1}}, \quad x_{p+1} = u^{n_p}$. \end{center} In this article, we consider the case when $n_0, n_1, \ldots, n_p$ forms an arithmetic sequence, and give an explicit set of minimal generators for the derivation module $\mathrm{Der}_k(k[S])$. Further, we give an explicit formula for the Hilbert-Kunz multiplicity of the co-ordinate ring of a projective monomial curve.