2
The question is a little fuzzy, but this is what you’re looking for?
library(igraph)
g <- graph.full(n=10) # cria grafo completo, com 10 vertices
V(g) # 10 vertices numerados de 1 a 10
TRUE %in% (V(g) == 1) # o vertice 1 estah neste grafo?
[1] TRUE
TRUE %in% sum(V(g) == 15) # o vertice 15 estah neste grafo?
[1] FALSE