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A006011 a(n) = n^2*(n^2 - 1)/4.
(Formerly M3044)
20
0, 0, 3, 18, 60, 150, 315, 588, 1008, 1620, 2475, 3630, 5148, 7098, 9555, 12600, 16320, 20808, 26163, 32490, 39900, 48510, 58443, 69828, 82800, 97500, 114075, 132678, 153468, 176610, 202275, 230640, 261888, 296208, 333795, 374850, 419580, 468198 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Products of two consecutive triangular numbers (A000217).

a(n) = number of Lyndon words of length 4 on an n-letter alphabet. A Lyndon word is a primitive word that is lexicographically earliest in its cyclic rotation class. For example, a(2)=3 counts 1112, 1122, 1222. - David Callan, Nov 29 2007

For n >= 2 this is the second rightmost column of A163932. - Johannes W. Meijer, Oct 16 2009

Partial sums of A059270. - J. M. Bergot, Jun 27 2013

Using the integers, triangular numbers, and squares plot the points (A001477(n),A001477(n+1)), (A000217(n), A000217(n+1)), and (A000290(n),A000290(n+1) to create the vertices of a triangle. One-half the area of this triangle = a(n). - J. M. Bergot, Aug 01 2013

a(n) is the Wiener index of the triangular graph T(n+1). - Emeric Deutsch, Aug 26 2013

REFERENCES

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..10000

M. Azaola and F. Santos, The number of triangulations of the cyclic polytope C(n,n-4), Discrete Comput. Geom., 27 (2002), 29-48 (see Prop. 4.2(a)).

S. M. Losanitsch, Die Isomerie-Arten bei den Homologen der Paraffin-Reihe, Chem. Ber. 30 (1897), pp. 1917-1926.

S. M. Losanitsch, Die Isomerie-Arten bei den Homologen der Paraffin-Reihe, Chem. Ber. 30 (1897), 1917-1926. (Annotated scanned copy)

Eric Weisstein's World of Mathematics, Triangular Graph

Eric Weisstein's World of Mathematics, Wiener Index

Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).

FORMULA

G.f.: 3*(1 + x) / (1 - x)^5.

a(n) = (n-1)*n/2 * n*(n+1)/2 = A000217(n-1)*A000217(n) = 1/2*(n^2-1)*n^2/2 = 1/2*A000217(n^2-1). - Alexander Adamchuk, Apr 13 2006

a(n) = 3*A002415(n) = A047928(n-1)/4 = A083374(n-1)/2 = A008911(n)*3/2. - Zerinvary Lajos, May 09 2007

a(n) = (A126274(n)-A000537(n+1))/2. - Enrique Pérez Herrero, Mar 11 2013

ceil(sqrt(a(n)) + sqrt(a(n-1)))/2 = A000217(n). - Richard R. Forberg, Aug 14 2013

a(n) = Sum( i*(i^2+n), i=1..n-1 ) for n>1 (see Example section). [Bruno Berselli, Aug 29 2014]

Sum_{n>=2} 1/a(n) = 7 - 2*Pi^2/3 = 0.42026373260709425411... . - Vaclav Kotesovec, Apr 27 2016

EXAMPLE

After the zeros, the sequence is provided by the row sums of the triangle:

3;

4, 14;

5, 16, 39;

6, 18, 42, 84;

7, 20, 45, 88, 155;

8, 22, 48, 92, 160, 258;

9, 24, 51, 96, 165, 264, 399;

10, 26, 54, 100, 170, 270, 406, 584;

11, 28, 57, 104, 175, 276, 413, 592, 819;

12, 30, 60, 108, 180, 282, 420, 600, 828, 1110, etc.,

where T(r,c) = c*(c^2+r+1), with r = row index, c = column index, r>=c>0.

[Bruno Berselli, Aug 29 2014]

MAPLE

A006011 := proc(n)

    n^2*(n^2-1)/4 ;

end proc: # R. J. Mathar, Nov 29 2015

MATHEMATICA

Table[n^2 (n^2 - 1)/4, {n, 0, 38}]

Binomial[Range[20]^2, 2]/2 (* Eric W. Weisstein, Sep 08 2017 *)

LinearRecurrence[{5, -10, 10, -5, 1}, {0, 3, 18, 60, 150}, 20] (* Eric W. Weisstein, Sep 08 2017 *)

CoefficientList[Series[-3 x (1 + x)/(-1 + x)^5, {x, 0, 20}], x] (* Eric W. Weisstein, Sep 08 2017 *)

PROG

(MAGMA) [n^2*(n^2-1)/4: n in [0..40]]; // Vincenzo Librandi, Sep 14 2011

(PARI) a(n)=binomial(n^2, 2)/2 \\ Charles R Greathouse IV, Jun 27 2013

CROSSREFS

Thrice A002415. Row 4 of A074650.

Cf. A002415, A008911, A047928, A083374, A228317

A column of A124428.

Sequence in context: A190313 A139362 A012763 * A012779 A074439 A299031

Adjacent sequences:  A006008 A006009 A006010 * A006012 A006013 A006014

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane, Simon Plouffe

STATUS

approved

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Last modified February 24 06:13 EST 2018. Contains 299597 sequences. (Running on oeis4.)