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.
Categories: math.AC