Friendship graph

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Friendship graph
Friendship graph 8.svg
The friendship graph F8.
Vertices 2n+1
Edges 3n
Radius 1
Diameter 2
Girth 3
Chromatic number 3
Chromatic index 2n
Properties Unit distance
Notation Fn
The friendship graphs F2, F3 and F4.

In the mathematical field of graph theory, the friendship graph (or Dutch windmill graph or n-fan) Fn is a planar undirected graph with 2n+1 vertices and 3n edges.[1]

The friendship graph Fn can be constructed by joining n copies of the cycle graph C3 with a common vertex.[2]

By construction, the friendship graph Fn is isomorphic to the windmill graph Wd(3,n). It is unit distance with girth 3, diameter 2 and radius 1. The graph F2 is isomorphic to the butterfly graph.

Friendship theorem[edit]

The friendship theorem of Paul Erdős, Alfréd Rényi, and Vera T. Sós (1966)[3] states that the finite graphs with the property that every two vertices have exactly one neighbor in common are exactly the friendship graphs. Informally, if a group of people has the property that every pair of people has exactly one friend in common, then there must be one person who is a friend to all the others. However, for infinite graphs, there can be many different graphs with the same cardinality that have this property.[4]

A combinatorial proof of the friendship theorem was given by Mertzios and Unger.[5] Another proof was given by Craig Huneke.[6]

Labeling and colouring[edit]

The friendship graph has chromatic number 3 and chromatic index 2n. Its chromatic polynomial can be deduced from the chromatic polynomial of the cycle graph C3 and is equal to .

The friendship graph Fn is edge-graceful if and only if n is odd. It is graceful if and only if n ≡ 0 (mod 4) or n ≡ 1 (mod 4).[7][8]

Every friendship graph is factor-critical.

Extremal graph theory[edit]

According to extremal graph theory, every graph with sufficiently many edges (relative to its number of vertices) must contain a -fan as a subgraph. More specifically, this is true for an -vertex graph if the number of edges is

where is if is odd, and is if is even. These bounds generalize Turán's theorem on the number of edges in a triangle-free graph, and they are the best possible bounds for this problem, in that for any smaller number of edges there exist graphs that do not contain a -fan.[9]


  1. ^ Weisstein, Eric W. "Dutch Windmill Graph". MathWorld. 
  2. ^ Gallian, J. A. (January 3, 2007), "Dynamic Survey DS6: Graph Labeling" (PDF), Electronic J. Combinatorics, DS6, 1-58 .
  3. ^ Erdős, Paul; Rényi, Alfréd; Sós, Vera T. (1966), "On a problem of graph theory" (PDF), Studia Sci. Math. Hungar., 1: 215–235 .
  4. ^ Chvátal, Václav; Kotzig, Anton; Rosenberg, Ivo G.; Davies, Roy O. (1976), "There are friendship graphs of cardinal ", Canadian Mathematical Bulletin, 19 (4): 431–433, doi:10.4153/cmb-1976-064-1 .
  5. ^ Mertzios, George; Walter Unger (2008), "The friendship problem on graphs" (PDF), Relations, Orders and Graphs: Interaction with Computer Science 
  6. ^ Huneke, Craig (1 January 2002), "The Friendship Theorem", The American Mathematical Monthly, 109 (2): 192–194, doi:10.2307/2695332, JSTOR 2695332 
  7. ^ Bermond, J.-C.; Brouwer, A. E.; Germa, A. (1978), "Systèmes de triplets et différences associées", Problèmes Combinatoires et Théorie des Graphes (Univ. Orsay, 1976), Colloq. Intern. du CNRS, 260, CNRS, Paris, pp. 35–38, MR 0539936 .
  8. ^ Bermond, J.-C.; Kotzig, A.; Turgeon, J. (1978), "On a combinatorial problem of antennas in radioastronomy", Combinatorics (Proc. Fifth Hungarian Colloq., Keszthely, 1976), Vol. I, Colloq. Math. Soc. János Bolyai, 18, North-Holland, Amsterdam-New York, pp. 135–149, MR 0519261 .
  9. ^ Erdős, P.; Füredi, Z.; Gould, R. J.; Gunderson, D. S. (1995), "Extremal graphs for intersecting triangles", Journal of Combinatorial Theory, Series B, 64 (1): 89–100, doi:10.1006/jctb.1995.1026, MR 1328293 .