- Source: Rainbow coloring
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
G
{\displaystyle G}
is the minimum number of colors needed to rainbow-connect
G
{\displaystyle G}
, and is denoted by
rc
(
G
)
{\displaystyle {\text{rc}}(G)}
. Similarly, the strong rainbow connection number of a graph
G
{\displaystyle G}
is the minimum number of colors needed to strongly rainbow-connect
G
{\displaystyle G}
, and is denoted by
src
(
G
)
{\displaystyle {\text{src}}(G)}
. 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
G
{\displaystyle G}
, we need at least
diam
(
G
)
{\displaystyle {\text{diam}}(G)}
colors, where
diam
(
G
)
{\displaystyle {\text{diam}}(G)}
is the diameter of
G
{\displaystyle G}
(i.e. the length of the longest shortest path). On the other hand, we can never use more than
m
{\displaystyle m}
colors, where
m
{\displaystyle m}
denotes the number of edges in
G
{\displaystyle G}
. Finally, because each strongly rainbow-connected graph is rainbow-connected, we have that
diam
(
G
)
≤
rc
(
G
)
≤
src
(
G
)
≤
m
{\displaystyle {\text{diam}}(G)\leq {\text{rc}}(G)\leq {\text{src}}(G)\leq m}
.
The following are the extremal cases:
rc
(
G
)
=
src
(
G
)
=
1
{\displaystyle {\text{rc}}(G)={\text{src}}(G)=1}
if and only if
G
{\displaystyle G}
is a complete graph.
rc
(
G
)
=
src
(
G
)
=
m
{\displaystyle {\text{rc}}(G)={\text{src}}(G)=m}
if and only if
G
{\displaystyle G}
is a tree.
The above shows that in terms of the number of vertices, the upper bound
rc
(
G
)
≤
n
−
1
{\displaystyle {\text{rc}}(G)\leq n-1}
is the best possible in general. In fact, a rainbow coloring using
n
−
1
{\displaystyle n-1}
colors can be constructed by coloring the edges of a spanning tree of
G
{\displaystyle G}
in distinct colors. The remaining uncolored edges are colored arbitrarily, without introducing new colors. When
G
{\displaystyle G}
is 2-connected, we have that
rc
(
G
)
≤
⌈
n
/
2
⌉
{\displaystyle {\text{rc}}(G)\leq \lceil n/2\rceil }
. 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
(
C
n
)
=
src
(
C
n
)
=
⌈
n
/
2
⌉
{\displaystyle {\text{rc}}(C_{n})={\text{src}}(C_{n})=\lceil n/2\rceil }
, for each integer
n
≥
4
{\displaystyle n\geq 4}
, where
C
n
{\displaystyle C_{n}}
is the cycle graph.
rc
(
W
n
)
=
3
{\displaystyle {\text{rc}}(W_{n})=3}
, for each integer
n
≥
7
{\displaystyle n\geq 7}
, and
src
(
W
n
)
=
⌈
n
/
3
⌉
{\displaystyle {\text{src}}(W_{n})=\lceil n/3\rceil }
, for
n
≥
3
{\displaystyle n\geq 3}
, where
W
n
{\displaystyle W_{n}}
is the wheel graph.
Complexity
The problem of deciding whether
rc
(
G
)
=
2
{\displaystyle {\text{rc}}(G)=2}
for a given graph
G
{\displaystyle G}
is NP-complete. Because
rc
(
G
)
=
2
{\displaystyle {\text{rc}}(G)=2}
if and only if
src
(
G
)
=
2
{\displaystyle {\text{src}}(G)=2}
, it follows that deciding if
src
(
G
)
=
2
{\displaystyle {\text{src}}(G)=2}
is NP-complete for a given graph
G
{\displaystyle G}
.
Variants and generalizations
Chartrand, Okamoto and Zhang generalized the rainbow connection number as follows. Let
G
{\displaystyle G}
be an edge-colored nontrivial connected graph of order
n
{\displaystyle n}
. A tree
T
{\displaystyle T}
is a rainbow tree if no two edges of
T
{\displaystyle T}
are assigned the same color. Let
k
{\displaystyle k}
be a fixed integer with
2
≤
k
≤
n
{\displaystyle 2\leq k\leq n}
. An edge coloring of
G
{\displaystyle G}
is called a
k
{\displaystyle k}
-rainbow coloring if for every set
S
{\displaystyle S}
of
k
{\displaystyle k}
vertices of
G
{\displaystyle G}
, there is a rainbow tree in
G
{\displaystyle G}
containing the vertices of
S
{\displaystyle S}
. The
k
{\displaystyle k}
-rainbow index
rx
k
(
G
)
{\displaystyle {\text{rx}}_{k}(G)}
of
G
{\displaystyle G}
is the minimum number of colors needed in a
k
{\displaystyle k}
-rainbow coloring of
G
{\displaystyle G}
. A
k
{\displaystyle k}
-rainbow coloring using
rx
k
(
G
)
{\displaystyle {\text{rx}}_{k}(G)}
colors is called a minimum
k
{\displaystyle k}
-rainbow coloring. Thus
rx
2
(
G
)
{\displaystyle {\text{rx}}_{2}(G)}
is the rainbow connection number of
G
{\displaystyle G}
.
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
G
{\displaystyle G}
, denoted by
rvc
(
G
)
{\displaystyle {\text{rvc}}(G)}
, is the minimum number of colors needed to color
G
{\displaystyle G}
such that for each pair of vertices, there is a path connecting them whose internal vertices are assigned distinct colors.
See also
Rainbow matching
Notes
References
Chartrand, Gary; Johns, Garry L.; McKeon, Kathleen A.; Zhang, Ping (2008), "Rainbow connection in graphs", Mathematica Bohemica, 133 (1): 85–98, doi:10.21136/MB.2008.133947.
Chartrand, Gary; Okamoto, Futaba; Zhang, Ping (2010), "Rainbow trees in graphs and generalized connectivity", Networks, 55 (4): NA, doi:10.1002/net.20339, S2CID 7505197.
Chakraborty, Sourav; Fischer, Eldar; Matsliah, Arie; Yuster, Raphael (2011), "Hardness and algorithms for rainbow connection", Journal of Combinatorial Optimization, 21 (3): 330–347, arXiv:0809.2493, doi:10.1007/s10878-009-9250-9, S2CID 10874392.
Krivelevich, Michael; Yuster, Raphael (2010), "The Rainbow Connection of a Graph Is (at Most) Reciprocal to Its Minimum Degree", Journal of Graph Theory, 63 (3): 185–191, doi:10.1002/jgt.20418.
Li, Xueliang; Shi, Yongtang; Sun, Yuefang (2013), "Rainbow Connections of Graphs: A Survey", Graphs and Combinatorics, 29 (1): 1–38, arXiv:1101.5747, doi:10.1007/s00373-012-1243-2, S2CID 253898232.
Li, Xueliang; Sun, Yuefang (2012), Rainbow connections of graphs, Springer, p. 103, ISBN 978-1-4614-3119-0.
Ekstein, Jan; Holub, Přemysl; Kaiser, Tomáš; Koch, Maria; Camacho, Stephan Matos; Ryjáček, Zdeněk; Schiermeyer, Ingo (2013), "The rainbow connection number of 2-connected graphs", Discrete Mathematics, 313 (19): 1884–1892, arXiv:1110.5736, doi:10.1016/j.disc.2012.04.022, S2CID 16596310.