Generalized Hilbert-Kunz function in graded dimension two

We prove that the generalized Hilbert-Kunz function of a graded module $M$ over a two-dimensional standard graded normal $K$-domain over an algebraically closed field $K$ of prime characteristic $p$ has the form $gHK(M,q)=e_{gHK}(M)q^2+\gamma (q)$, with rational generalized Hilbert-Kunz multiplicity $e_{gHK}(M)$ and a bounded function $\gamma (q)$. Moreover we prove that if $R$ is a $\mathbb{Z}$-algebra, the limit for $p\to +\mathrm{\infty }$ of the generalized Hilbert-Kunz multiplicity $e_{gHK}^{R_p}(M_p)$ over the fibers $R_p$ exists and it is a rational number.