Rainbow coloring explained
In graph theory, a path in an edge-colored graph is said to be rainbow if no color repeats on it. A graph is said to be rainbow-connected (or rainbow colored) if there is a rainbow path between each pair of its vertices. If there is a rainbow shortest path between each pair of vertices, the graph is said to be strongly rainbow-connected (or strongly rainbow colored).
Definitions and bounds
The rainbow connection number of a graph
is the minimum number of colors needed to rainbow-connect
, and is denoted by
. Similarly, the
strong rainbow connection number of a graph
is the minimum number of colors needed to strongly rainbow-connect
, and is denoted by
. Clearly, each strong rainbow coloring is also a rainbow coloring, while the converse is not true in general.
It is easy to observe that to rainbow-connect any connected graph
, we need at least
colors, where
is the
diameter of
(i.e. the length of the longest shortest path). On the other hand, we can never use more than
colors, where
denotes the number of edges in
. Finally, because each strongly rainbow-connected graph is rainbow-connected, we have that
diam(G)\leqrc(G)\leqsrc(G)\leqm
.
The following are the extremal cases:
if and only if
is a
complete graph.
if and only if
is a
tree.
The above shows that in terms of the number of vertices, the upper bound
is the best possible in general. In fact, a rainbow coloring using
colors can be constructed by coloring the edges of a spanning tree of
in distinct colors. The remaining uncolored edges are colored arbitrarily, without introducing new colors. When
is 2-connected, we have that
. Moreover, this is tight as witnessed by e.g. odd cycles.
Exact rainbow or strong rainbow connection numbers
The rainbow or the strong rainbow connection number has been determined for some structured graph classes:
rc(Cn)=src(Cn)=\lceiln/2\rceil
, for each integer
, where
is the
cycle graph.
, for each integer
, and
, for
, where
is the
wheel graph.
Complexity
The problem of deciding whether
for a given graph
is
NP-complete. Because
if and only if
, it follows that deciding if
is NP-complete for a given graph
.
Variants and generalizations
Chartrand, Okamoto and Zhang generalized the rainbow connection number as follows. Let
be an edge-colored nontrivial connected graph of order
. A tree
is a
rainbow tree if no two edges of
are assigned the same color. Let
be a fixed integer with
. An edge coloring of
is called a
-rainbow coloring if for every set
of
vertices of
, there is a rainbow tree in
containing the vertices of
. The
-rainbow index
of
is the minimum number of colors needed in a
-rainbow coloring of
. A
-rainbow coloring using
colors is called a
minimum
-rainbow coloring. Thus
is the rainbow connection number of
.
Rainbow connection has also been studied in vertex-colored graphs. This concept was introduced by Krivelevich and Yuster.Here, the rainbow vertex-connection number of a graph
, denoted by
, is the minimum number of colors needed to color
such that for each pair of vertices, there is a path connecting them whose internal vertices are assigned distinct colors.
See also
References