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 A050534 Tritriangular numbers: a(n) = binomial(binomial(n,2),2) = n(n + 1)(n - 1)(n - 2)/8. 47
 0, 0, 0, 3, 15, 45, 105, 210, 378, 630, 990, 1485, 2145, 3003, 4095, 5460, 7140, 9180, 11628, 14535, 17955, 21945, 26565, 31878, 37950, 44850, 52650, 61425, 71253, 82215, 94395, 107880, 122760, 139128, 157080, 176715, 198135, 221445, 246753 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS "There are n straight lines in a plane, no two of which are parallel and no three of which are concurrent. Their points of intersection being joined, show that the number of new lines drawn is (1/8)n(n-1)(n-2)(n-3)." - The American Mathematical Monthly 22(1915) 130 by C. N. Schmall Several different versions of this sequence are possible, beginning with either one, two or three 0's. If Y is a 3-subset of an n-set X then, for n>=6, a(n-4) is the number of (n-6)-subsets of X which have exactly one element in common with Y. - Milan Janjic, Dec 28 2007 Number of distinct ways to select 2 pairs of objects from a set of n+1 objects, when order doesn't matter. For example, with n = 3 (4 objects), the 3 possibilities are (12)(34), (13)(24), and (14)(23). - Brian Parsonnet, Jan 03 2012 Partial sums of A027480. - J. M. Bergot, Jul 09 2013 For the set {1,2,...,n}, the sum of the 2 smallest elements of all subsets with 3 elements is a(n) (see Bulut et al. link). - Serhat Bulut, Jan 20 2015 a(n) is also the number of subgroups of S_{n+1} (the symmetric group on n+1 elements) that are isomorphic to D_4 (the dihedral group of order 8). - Geoffrey Critzer, Sep 13 2015 a(n) is the coefficient of x1^(n-3)*x2^2 in exponential Bell polynomial B_{n+1}(x1,x2,...) (number of ways to select 2 pairs among n+1 objects, see above), hence its link with A000292 and A001296 (see formula). - Cyril Damamme, Feb 26 2018 Also the number of 4-cycles in the complete graph K_{n+1}. - Eric W. Weisstein, Mar 13 2018 Number of chiral pairs of colorings of the 4 edges or vertices of a square using n or fewer colors. Each member of a chiral pair is a reflection, but not a rotation, of the other. - Robert A. Russell, Oct 20 2020 REFERENCES A. T. Benjamin and J. J. Quinn, Proofs that really count: the art of combinatorial proof, M.A.A. 2003, id. 154. L. Comtet, Advanced Combinatorics, Reidel, 1974, Problem 1, page 72. R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.5, case k=2. LINKS William A. Tedeschi, Table of n, a(n) for n = 0..10000 Serhat Bulut and Oktay Erkan Temizkan, Subset Sum Problem, Jan 20 2015. A. Burstein, S. Kitaev, T. Mansour, Partially ordered patterns and their combinatorial interpretations, PU. M. A. Vol. 19 (2008), No. 2-3, pp. 27-38. L. H. Kauffman, Non-Commutative Worlds-Classical Constraints, Relativity and the Bianchi Identity, arXiv preprint arXiv:1109.1085 [math-ph], 2011. (See Appendix) Alexander Kreinin, Integer Sequences and Laplace Continued Fraction, Journal of Integer Sequences, Vol. 19 (2016), #16.6.2. Frank Ruskey and Jennifer Woodcock, The Rand and block distances of pairs of set partitions, in Combinatorial Algorithms, 287-299, Lecture Notes in Comput. Sci., 7056, Springer, Heidelberg, 2011. C. N. Schmall, Problem 432, The American Mathematical Monthly 22(1915) 130. Eric Weisstein's World of Mathematics, Complete Graph Eric Weisstein's World of Mathematics, Graph Cycle Eric Weisstein's World of Mathematics, Tritriangular Number Chai Wah Wu, Graphs whose normalized Laplacian matrices are separable as density matrices in quantum mechanics, arXiv:1407.5663 [quant-ph], 2014. Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1). FORMULA a(n) = 3*binomial(n+1, 4) = 3*A000332(n+1). Recurrence: a(n) = 5*a(n-1)-10*a(n-2)+10*a(n-3)-5*a(n-4)+a(n-5). G.f.: 3*x^3 / (1-x)^5. - Vladeta Jovovic, May 03 2002 a(n+1) = T(T(n))-T(n); a(n+2) = T(T(n)+n) where T is A000217. - Jon Perry, Jun 11 2003 a(n+1) = T(n)^2-T(T(n)) where T is A000217. - Jon Perry, Jul 23 2003 a(n) = T(T(n-1)-1) where T is A000217. - Jon E. Schoenfield, Dec 14 2014 a(n) = 3*C(n, 4) + 3*C(n, 3), for n>3. a(n) = Sum[(k*(k-1)*(k-2)),{k,1,n}]/2. a(n) = A033487(n-2)/2, n>1. a(n) = C(n-1,2)*C(n+1,2)/2, n>2. - Alexander Adamchuk, Apr 11 2006 a(n) = A052762(n+1)/8. - Zerinvary Lajos, Apr 26 2007 a(n) = (4x^4 - 4x^3 - x^2 + x)/2 where x = floor(n/2)*(-1)^n for n >= 0. - William A. Tedeschi, Aug 24 2010 E.g.f.: x^3*exp(x)*(4+x)/8. - Robert Israel, Nov 01 2015 a(n) = Sum_{k=1..n} Sum_{i=1..k} (n-i-1)*(n-k). - Wesley Ivan Hurt, Sep 12 2017 a(n) = A001296(n-1) - A000292(n-1). - Cyril Damamme, Feb 26 2018 Sum_{n>=3} 1/a(n) = 4/9. - Vaclav Kotesovec, May 01 2018 a(n) = A006528(n) - A002817(n) = (A006528(n) - A002411(n)) / 2 = A002817(n) - A002411(n). - Robert A. Russell, Oct 20 2020 EXAMPLE For a(3)=3, the chiral pairs of square colorings are AABC-AACB, ABBC-ACBB, and ABCC-ACCB. - Robert A. Russell, Oct 20 2020 MAPLE [seq(binomial(n+1, 4)*3, n=0..40)]; # Zerinvary Lajos, Jul 18 2006 MATHEMATICA Table[Binomial[Binomial[n, 2], 2], {n, 0, 50}] (* Stefan Steinerberger, Apr 08 2006 *) LinearRecurrence[{5, -10, 10, -5, 1}, {0, 0, 0, 3, 15}, 40] (* Harvey P. Dale, Dec 14 2011 *) (* Start from Eric W. Weisstein, Mar 13 2018 *) Binomial[Binomial[Range[0, 20], 2], 2] Nest[Binomial[#, 2] &, Range[0, 20], 2] Nest[PolygonalNumber[# - 1] &, Range[0, 20], 2] CoefficientList[Series[3 x^3/(1 - x)^5, {x, 0, 20}], x] (* End *) PROG (Sage) [(binomial(binomial(n, 2), 2)) for n in range(0, 39)] # Zerinvary Lajos, Nov 30 2009 (PARI) a(n)=n*(n+1)*(n-1)*(n-2)/8 \\ Charles R Greathouse IV, Nov 20 2012 (MAGMA) [3*Binomial(n+1, 4): n in [0..40]]; // Vincenzo Librandi, Feb 14 2015 (PARI) x='x+O('x^100); concat([0, 0, 0], Vec(3*x^3/(1-x)^5)) \\ Altug Alkan, Nov 01 2015 (GAP) List([0..40], n->3*Binomial(n+1, 4)); # Muniru A Asiru, Mar 20 2018 CROSSREFS Cf. A000217, A000332, A033487, A107394, A033487, A034827, A210569, Second column of triangle A001498. Cf. similar sequences listed in A241765. Cf. (square colorings) A006528 (oriented), A002817 (unoriented), A002411 (achiral), Row 2 of A325006 (orthoplex facets, orthotope vertices) and A337409 (orthotope edges, orthoplex ridges). Row 4 of A293496 (cycles of n colors using k or fewer colors). Sequence in context: A112810 A334078 A094191 * A048099 A030505 A301632 Adjacent sequences:  A050531 A050532 A050533 * A050535 A050536 A050537 KEYWORD easy,nice,nonn AUTHOR Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 29 1999 EXTENSIONS Additional comments from Antreas P. Hatzipolakis, May 03 2002 STATUS approved

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Last modified April 11 10:11 EDT 2021. Contains 342886 sequences. (Running on oeis4.)