On invariant of the regularity index of fat points

We prove invariant of the regularity index of fat points under changes of the linear subspace containing the support of the fat points. Then we show that Segre's bound is attained by any set of s non-degenerate equimultiple fat points in $\mathbb P^n$, $s\le n+3$. We also give an example showing that there always exists a set of n+4 non-degenerate equimultiple fat points in $\mathbb P^n$ such that Segre's bound is not attained.