# **Impedance key entities**

Key entities of the argument and results of the Impedance functions are:

**Node**: a set of abstract locations, connected by links. In graph theory a node is often called a vertex.

**Link**: a set of connections between nodes identified by two mappings, *f*_{1} : Link → Node, and *f*_{2} : Link → Node indicating the start and end node for each link.In graph theory a link is often called an edge

**OrgZone**: a set of origins

**DstZone**: a set of destinations

**startPoint**: a set of combinations of origins and nodes (thus allowing multiple nodes per origin and multiple origins per node).

**endPoint**: a set of combinations of destinations and nodes (thus allowing multiple nodes per destination and multiple destinations per node).

Depending on the given arguments, it may well be that these entities are not distinctive, for example, OrgZone may be equal to DstZone.

Relevant values-unit are:

**Impedance**: a non-negative numerical expression of travel resistance for which comparison and addition make sense.

**Impedance2**: a non-negative numerical expression of an alternative travel resistance for which addition makes sense. If no alternative impedance is given, Impedance2 is set equal to Impedance.

**Mass**: a non-negative numerical expression of OrgZone-related demand or DstZone-related supply