In my definition, this is a closed walk? If yes, how can I enumerate the vertices? In this picture, who is $v_1, v_2,...$?
Definition: A closed walk in a graph is defined to be ordered collection of vertices $(v_1,v_2,...,v_n)$ such that $v_i$ and $v_{i+1}$ are neighbours for all $1 \leq i \leq n-1$, and $v_n$ and $v_1$ are neighbours. Note that the walk is allowed to visit the same vertex or traverse the same edge on more than one occasion.