In the
mathematical field of
graph theory, the term "null graph" may refer either to the
order
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
-
zero graph, or alternatively, to any edgeless graph (the latter is sometimes called an "empty graph").
Order-zero graph
The order-zero graph, , is the unique graph having no
vertices (hence its order is zero). It follows that also has no
edges
Edge or EDGE may refer to:
Technology Computing
* Edge computing, a network load-balancing system
* Edge device, an entry point to a computer network
* Adobe Edge, a graphical development application
* Microsoft Edge, a web browser developed by ...
. Thus the null graph is a
regular graph of degree zero. Some authors exclude from consideration as a graph (either by definition, or more simply as a matter of convenience). Whether including as a valid graph is useful depends on context. On the positive side, follows naturally from the usual
set-theoretic definitions of a graph (it is the
ordered pair
In mathematics, an ordered pair (''a'', ''b'') is a pair of objects. The order in which the objects appear in the pair is significant: the ordered pair (''a'', ''b'') is different from the ordered pair (''b'', ''a'') unless ''a'' = ''b''. (In con ...
for which the vertex and edge sets, and , are both
empty), in
proofs it serves as a natural base case for
mathematical induction, and similarly, in
recursively defined data structure
In computer science, a data structure is a data organization, management, and storage format that is usually chosen for efficient access to data. More precisely, a data structure is a collection of data values, the relationships among them, a ...
s is useful for defining the base case for recursion (by treating the null
tree as the