# Bigraph

A bigraph (often used in the plural bigraphs) can be modelled as the superposition of a graph (the link graph) and a set of trees (the place graph).[1][2]

Each node of the bigraph is part of a graph and also part of some tree that describes how the nodes are nested. Bigraphs can be conveniently and formally displayed as diagrams.[1] They have applications in the modelling of distributed systems for ubiquitous computing and can be used to describe mobile interactions. They have also been used by Robin Milner in an attempt to subsume Calculus of Communicating Systems (CCS) and π-calculus.[2] They have been studied in the context of category theory.[3]

## Anatomy of a bigraph

Aside from nodes and (hyper-)edges, a bigraph may have associated with it one or more regions which are roots in the place forest, and zero or more holes in the place graph, into which other bigraph regions may be inserted. Similarly, to nodes we may assign controls that define identities and an arity (the number of ports for a given node to which link-graph edges may connect). These controls are drawn from a bigraph signature. In the link graph we define inner and outer names, which define the connection points at which coincident names may be fused to form a single link.

## Foundations

A bigraph is a 5-tuple:

$(V,E,ctrl,prnt,link) : \langle k,X \rangle \to \langle m,Y \rangle,$

where $V$ is a set of nodes, $E$ is a set of edges, $ctrl$ is the control map that assigns controls to nodes, $prnt$ is the parent map that defines the nesting of nodes, and $link$ is the link map that defines the link structure.

The notation $\langle k,X \rangle \to \langle m,Y \rangle$ indicates that the bigraph has $k$ holes (sites) and a set of inner names $X$ and $m$ regions, with a set of outer names $Y$. These are respectively known as the inner and outer interfaces of the bigraph.

Formally speaking, each bigraph is an arrow in a symmetric partial monoidal category (usually abbreviated spm-category) in which the objects are these interfaces.[4] As a result, the composition of bigraphs is definable in terms of the composition of arrows in the category.