On minimum $ (K_{1,r};k) $-vertex stable graphs on the exact number of vertices

Abstract

A graph $ G $ is said to be $ (H;k) $-vertex stable if $ G $ contains a~subgraph isomorphic to $ H $ even after removing any $ k $ of its vertices alongside with their incident edges. We will denote by $ \text{stab}(H;k) $ the minimum size among sizes of all $ (H;k) $-vertex stable graphs. In this paper we consider a~case where the structure $ H $ is a~star graph $ K_{1,r} $ and the the number of vertices in $ G $ is exact, \ie equal to $ 1 + r + k $. We will show that under the above assumptions $ \text{stab}(K_{1,r};k) $ equals either $ \frac{1}{2}(k + 1)(2r + k) $, $ \frac{1}{2}\big((r + k)^{2} - 1\big) $ or $ \frac{1}{2}(r + k)^{2} $. Moreover, we will characterize all the extremal graphs.