... undirected, and simple. The vertex set and edge s et of a graph Gare denoted by V (G) and E(G). For vertices v, w ∈ V (G), we write v ∼ w if vw ∈ E(G), and v ∼ wif vw ∈ E(G). For S ⊆ V (G), ... G∗∼=PD+1 and for some k ∈ [2, D − 1], the vertices u∗k−1 and u∗k+1of G∗are both not of type (1). Then u∗k−1 and u∗k+1are both of type (N).Proof. Let x and y be twins of uk−1 and uk+1respectively. ... of uk−1, uk and w (if they exist), and another vertex of G. Let xk−1, xk, and y respectively be twinsof uk−1, uk and w (if they exist). Hence xk−1∼ uk−1, xk∼ uk and y ∼ w. Consequently,uk−1,...