On the bounds of feedback numbers of (n, k)-star graphs

The feedback number of a graph G is the minimum number of vertices whose removal from G results in an acyclic subgraph. We use f(n,k) to denote the feedback number of the (n,k)-star graph S"n","k and p(n,k) the number of k-permutations of an n-element set. This paper proves thatp(n,k)-2(k-1)!(nk-1)=