In order to design a systematic way to determine which neighboring RPDAGs arise from the inclusion or the removal of an edge in an RPDAG, it is sufficient to consider some local parameters of the two nodes to be connected. First, some additional notation is introduced. If represents the cardinality of a set, given a node in a PDAG , we define:

- ,
- ,

- if (hence )

Luis Miguel de Campos Ibáñez 2003-05-30