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A152947 a(n) = 1+(n-2)*(n-1)/2. 11
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-Fran├žois Alcover (this remark is due to Paul Curtz), Jun 20 2016

LINKS

Table of n, a(n) for n=1..54.

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.

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 xrange(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,changed

AUTHOR

Vladimir Joseph Stephan Orlovsky, Dec 15 2008

STATUS

approved

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Last modified May 23 06:42 EDT 2017. Contains 286909 sequences.