图G的平方图G^2是以V(G)作为它的点集,两个点在G^2中相邻当且仅当它们在G中的距离至多为2.证明了:若G是一个最大度△≥6的外平面图,则G^2的点荫度va(G^2)=[(Δ+1)/2];特别地,一棵树T的平方图T^2的点荫度va(T^2)=[(Δ+1)/2].
The square graph G^2 of agraph G is the graph defined on the vertex set V(G) such that two vertices are adjacent in G^2 if and only if the distance between them is at most 2. It is proved that if G is an outerplanar graph with maximum degree Δ≥ 6, then the vertex arbericity va(G^2) of G^2 is [(Δ+1)/2]; In particular, if T is a tree, the vertex arbericity va(T^2) = [(Δ+1)/2].