The composition graph of a family of n+1 disjoint graphs ${H_i:0 ≤ i ≤ n}$ is the graph H obtained by substituting the n vertices of H₀ respectively by the graphs H₁,H₂,...,Hₙ. If H has some hereditary property P, then necessarily all its factors enjoy the same property. For some sort of graphs it is sufficient that all factors ${H_i: 0 ≤ i ≤ n}$ have a certain common P to endow H with this P. For instance, it is known that the composition graph of a family of perfect graphs is also a perfect graph (B. Bollobas, 1978), and the composition graph of a family of comparability graphs is a comparability graph as well (M.C. Golumbic, 1980). In this paper we show that the composition graph of a family of co-graphs (i.e., P₄-free graphs), is also a co-graph, whereas for θ₁-perfect graphs (i.e., P₄-free and C₄-free graphs) and for threshold graphs (i.e., P₄-free, C₄-free and 2K₂-free graphs), the corresponding factors ${H_i:0 ≤ i ≤ n}$ have to be equipped with some special structure.
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