Is there a term for this graph construction? I replace several vertices by a single one.

Question

I want to study a graph $\mathcal G=(\mathcal V,\mathcal E)$, which emerges from a simple graph $G=(V,E)$ by considering certain vertex sets in $G$ as single vertices. In detail: I choose a partition $V_1\,\dot\cup\,\cdots\,\dot\cup\, V_n=V$ and define

$$\mathcal V=\{V_1,...,V_n\},\qquad \mathcal E=\{\{V_1,V_2\}\mid \text{$E$ contains an edge between $V_1$ and $V_2$}\}.$$

To me, $\mathcal G$ looks like a graph minor of $G$, but 1) we can contract also edges which are not there, 2) I am not allowed to delete edges. Is there a term for such a process or such a graph $\mathcal G$ (cluster graph, block graph, ...)?

Answer 1

The term you are looking for is Quotient Graph.

Answer 2

I would like to point out that this kind of construction is more general. As has been pointed out, this is a quotient graph.

In topology, if you have a topological space, you can create a new space by considering some points in the space to be the same. (For example, if you have $[0,1]$ with standard subspace topology, then you form a new space, say by considering $0$ and $1$ to be the same. This will be homeomorphic to a circle $S^1$)

In abstract algebra, you can often take quotients on your structure, which will amount to thinking of different elements as the same (here you have to be careful about how you do this). For example, if you have a group and a normal subgroup, then you can get a quotient group where the cosets are the group elements. You can think of these cosets as the equivalence classes.