# Algebra of subgraphs

```
using Catlab.Graphs, Catlab.Graphics
using Catlab.Theories, Catlab.CategoricalAlgebra
```

A *subgraph* of a graph $G$ is a monomorphism $A \rightarrowtail G$. Because the category of graphs is a presheaf topos, its subobjects have a rich algebraic structure, which we will explore in this vignette.

Throughout the vignette, we will work with subgraphs of the following graph.

```
G = cycle_graph(Graph, 4) ⊕ path_graph(Graph, 2) ⊕ cycle_graph(Graph, 1)
add_edge!(G, 3, add_vertex!(G))
to_graphviz(G, node_labels=true, edge_labels=true)
```

## Meet and join

The basic operations of meet or intersection ($\wedge$), join or union ($\vee$), top or maximum ($\top$), bottom or minimum ($\bot$) are all computed pointwise: separately on vertices and edges.

Consider the following two subgraphs.

`(A = Subobject(G, V=1:4, E=[1,2,4])) |> to_graphviz`

`(B = Subobject(G, V=[2,3,4,7,8], E=[2,3,6,7])) |> to_graphviz`

The *join* is defined as left adjoint to the diagonal, making it the *least upper bound*:

\[A \vee B \leq C \qquad\text{iff}\qquad A \leq C \text{ and } B \leq C.\]

`A ∨ B |> to_graphviz`

Dually, the *meet* is defined as right adjoint to the diagonal, making it the *greatest lower bound*:

\[C \leq A \text{ and } C \leq B \qquad\text{iff}\qquad C \leq A \wedge B.\]

`A ∧ B |> to_graphviz`

## Implication and negation

The other operations, beginning with implication ($\Rightarrow$) and negation ($\neg$) are more interesting because they do not have pointwise formulas.

*Implication* is defined as the right adjoint to the meet:

\[C \wedge A \leq B \qquad\text{iff}\qquad C \leq A \Rightarrow B.\]

`(A ⟹ B) |> to_graphviz`

*Negation* is defined by setting $B = \bot$ in the above formula:

\[C \wedge A = \bot \qquad\text{iff}\qquad C \leq \neg A.\]

`¬A |> to_graphviz`

### Induced subgraph as a double negation

The logic of subgraphs, and of subobjects in presheaf toposes generally, is not classical. Specifically, subobjects form a Heyting algebra but not a Boolean algebra. This means that the law of excluded middle does not hold: in general, $\neg \neg A \neq A$.

Applying the double negation to a discrete subgraph gives the subgraph induced by those vertices.

`(C = Subobject(G, V=1:4)) |> to_graphviz`

`¬(¬C) |> to_graphviz`

## Subtraction and non

The subojects also form co-Heyting algebra and hence a bi-Heyting algebra.

*Subtraction* is defined dually to implication as the left adjoint to the join:

\[A \leq B \vee C \qquad\text{iff}\qquad A \setminus B \leq C.\]

`(A \ B) |> to_graphviz`

*Non* is defined by setting $A = \top$ in the above formula:

\[\top = B \vee C \qquad\text{iff}\qquad {\sim} B \leq C.\]

`~A |> to_graphviz`

### Boundary via non

A *boundary* operator can be defined using the non operator:

\[\partial A := A \wedge {\sim} A.\]

`(A ∧ ~A) |> to_graphviz`