Discuss the below:
Q: For n∈N, let G be the graph with vertex set {v_0,...,v_3n} defined by v_i↔v_j if and only if |i-j|≤2 and i+j is not divisible by 6.
a) Determine the blocks of G.
b) Prove that adding the edge v_0 v_3n to G creates a 4-critical graph.