OFFSET
5,1
COMMENTS
The Kneser graph K(n,2) is the graph whose vertices represent the 2-subsets of {1,2,...,n} and two vertices are connected if and only if they correspond to disjoint subsets.
K(n,2) is disconnected for n<=4.
K(5,2) is the Petersen graph.
The Kneser graph K(n,2) is distance-regular with intersection array [(n-2)*(n-3)/2, 2*(n-4); 1, (n-3)*(n-4)/2].
REFERENCES
R. Balakkrishnan, S. Francis Raj, The Wiener number of Kneser graphs, Discussiones Math, Graph Theory, 28, 2008, 219-228.
LINKS
Eric Weisstein's World of Mathematics, Kneser Graph.
Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).
FORMULA
a(n) = (1/8)*n*(n-1)*(n-2)*(n+9).
G.f.: 3*x^5*(35-100*x+115*x^2-62*x^3+13*x^4)/(1-x)^5.
The Hosoya-Wiener polynomial of K(n,2) is (1/8)*n*(n-1)*(n-2)*t*(n-3+4*t).
a(n) = 3*A095661(n-3). - R. J. Mathar, Aug 21 2013
MAPLE
a := proc (n) options operator, arrow: (1/8)*n*(n-1)*(n-2)*(n+9) end proc: seq(a(n), n = 5 .. 40);
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Emeric Deutsch, Aug 20 2013
STATUS
approved