This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A228317 The hyper-Wiener index of the triangular graph T(n) (n>=1). 2
 0, 0, 3, 21, 75, 195, 420, 798, 1386, 2250, 3465, 5115, 7293, 10101, 13650, 18060, 23460, 29988, 37791, 47025, 57855, 70455, 85008, 101706, 120750, 142350, 166725, 194103, 224721, 258825, 296670, 338520, 384648, 435336, 490875, 551565, 617715, 689643 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The triangular graph T(n) is the graph whose vertices represent the 2-subsets of {1,2,...,n} and two vertices are adjacent provided the corresponding 2-subsets have a nonempty intersection. The triangular graph T(n) is a strongly regular graph with parameters n(n-1)/2, 2(n-2), n-2, 4 (see the Brualdi et al. reference, Theorem 5.2.4). REFERENCES R. A. Brualdi, H. J. Ryser, Combinatorial Matrix Theory, Cambridge Univ. Press, 1992. LINKS Eric Weisstein's World of Mathematics, TriangularGraph. Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1). FORMULA a(n) = n*(n-1)*(n-2)*(3*n-5)/8. G.f.: 3*x^3*(1+2x)/(1-x)^5. The Hosoya-Wiener polynomial of T(n) is (1/8)n(n-1)(4+4(n-2)t+(n-2)(n-3)t^2). a(n) = 3*A001296(n-2). - R. J. Mathar, Mar 05 2017 MAPLE a := proc (n) options operator, arrow: (1/8)*n*(n-1)*(n-2)*(3*n-5) end proc: seq(a(n), n = 1 .. 38); CROSSREFS Cf. A006011 Sequence in context: A281008 A238193 A054646 * A322228 A109721 A067002 Adjacent sequences:  A228314 A228315 A228316 * A228318 A228319 A228320 KEYWORD nonn,easy AUTHOR Emeric Deutsch, Aug 26 2013 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified February 15 21:12 EST 2019. Contains 320138 sequences. (Running on oeis4.)