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 A152947 a(n) = 1 + (n-2)*(n-1)/2. 20
 1, 1, 2, 4, 7, 11, 16, 22, 29, 37, 46, 56, 67, 79, 92, 106, 121, 137, 154, 172, 191, 211, 232, 254, 277, 301, 326, 352, 379, 407, 436, 466, 497, 529, 562, 596, 631, 667, 704, 742, 781, 821, 862, 904, 947, 991, 1036, 1082, 1129, 1177, 1226, 1276, 1327, 1379 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The sequence is the sum of upward sloping terms in an infinite lower triangle with 1's in the leftmost column and the odd integers in all other columns. - Gary W. Adamson, Jan 29 2014 For n > 1, if Kruskal's algorithm is run on a weighted connected graph of n nodes, then a(n) is the maximum number of iterations required to reach a spanning tree. - Eric M. Schmidt, Jun 04 2016 It can be observed that A152947/A000079, whose reduced numerators are A213671, is identical to its inverse binomial transform (except for signs); this shows that it is an "autosequence" (more precisely, an autosequence of the second kind). - Jean-François Alcover (this remark is due to Paul Curtz), Jun 20 2016 LINKS Shawn A. Broyles, Table of n, a(n) for n = 1..1000 Christian Bean, Bjarki Gudmundsson, Henning Ulfarsson, Automatic discovery of structural rules of permutation classes, arXiv:1705.04109 [math.CO], 2017. H. Cheballah, S. Giraudo, R. Maurice, Combinatorial Hopf algebra structure on packed square matrices, arXiv preprint arXiv:1306.6605 [math.CO], 2013. Michael Dairyko, Samantha Tyner, Lara Pudwell, Casey Wynn, Non-contiguous pattern avoidance in binary trees, Electron. J. Combin. 19 (2012), no. 3, Paper 22, 21 pp. MR2967227. Lara Pudwell, Pattern avoidance in trees (slides from a talk, mentions many sequences), 2012. Franck Ramaharo, Statistics on some classes of knot shadows, arXiv:1802.07701 [math.CO], 2018. Franck Ramaharo, A generating polynomial for the pretzel knot, arXiv:1805.10680 [math.CO], 2018. Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = 1 + A000217(n-2) = A000124(n-2), n > 1. - R. J. Mathar, Jan 03 2009 a(n) = a(n-1) + n - 2 for n>1, a(1) = 1. - Vincenzo Librandi, Nov 26 2010 G.f.: -x*(1-2*x+2*x^2)/(x-1)^3. - R. J. Mathar, Nov 28 2010 From Ilya Gutkovskiy, Jun 04 2016: (Start) E.g.f.: (4 - 2*x + x^2)*exp(x)/2 - 2. Sum_{n>=1} 1/a(n) = 2*Pi*tanh(sqrt(7)*Pi/2)/sqrt(7) + 1 = A226985 + 1. (End) a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n>3. - Wesley Ivan Hurt, Jun 20 2016 MAPLE A152947:=n->1+(n-2)*(n-1)/2: seq(A152947(n), n=1..100); # Wesley Ivan Hurt, Jun 20 2016 MATHEMATICA Table[1 + (n^2 - 3n + 2)/2, {n, 50}] (* Alonso del Arte, Jan 30 2014 *) PROG (Sage) [1+binomial(n, 2) for n in range(0, 54)] # Zerinvary Lajos, Mar 12 2009 (MAGMA) [1+(n-2)*(n-1)/2: n in [1..60]]; // Klaus Brockhaus, Nov 28 2010 (PARI) a(n)=1+(n-2)*(n-1)/2 \\ Charles R Greathouse IV, Oct 07 2015 CROSSREFS Cf. A000124, A000217, A226985. Sequence in context: A025732 A025739 A000124 * A212369 A212368 A217838 Adjacent sequences:  A152944 A152945 A152946 * A152948 A152949 A152950 KEYWORD nonn,easy AUTHOR Vladimir Joseph Stephan Orlovsky, Dec 15 2008 STATUS approved

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Last modified May 26 22:58 EDT 2020. Contains 334634 sequences. (Running on oeis4.)