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 A002817 Doubly triangular numbers: a(n) = n*(n+1)*(n^2+n+2)/8. (Formerly M4141 N1718) 57
 0, 1, 6, 21, 55, 120, 231, 406, 666, 1035, 1540, 2211, 3081, 4186, 5565, 7260, 9316, 11781, 14706, 18145, 22155, 26796, 32131, 38226, 45150, 52975, 61776, 71631, 82621, 94830, 108345, 123256, 139656, 157641, 177310, 198765, 222111, 247456, 274911, 304590 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Number of inequivalent ways to color vertices of a square using <= n colors, allowing rotations and reflections. Group is dihedral group D_8 of order 8 with cycle index (1/8)*(x1^4 + 2*x4 + 3*x2^2 + 2*x1^2*x2); setting all x_i = n gives the formula a(n) = (1/8)*(n^4 + 2*n + 3*n^2 + 2*n^3). Number of semi-magic 3 X 3 squares with a line sum of n-1. That is, 3 X 3 matrices of nonnegative integers such that row sums and column sums are all equal to n-1. - [H. Gupta, Duke Math. J. 35 (1968) 653; Bell, 1970, page 279]. - Peter Bertok (peter(AT)bertok.com), Jan 12 2002. See A005045 for another version. Also the coefficient h_2 of x^{n-3} in the shelling polynomial h(x)=h_0*x^n-1 + h_1*x^n-2 + h_2*x^n-3 + ... + h_n-1 for the independence complex of the cycle matroid of the complete graph K_n on n vertices (n>=2) - Woong Kook (andrewk(AT)math.uri.edu), Nov 01 2006 If X is an n-set and Y a fixed 3-subset of X then a(n-4) is equal to the number of 5-subsets of X intersecting Y. - Milan Janjic, Jul 30 2007 Starting with offset 1 = binomial transform of [1, 5, 10, 9, 3, 0, 0, 0, ...]. - Gary W. Adamson, Aug 05 2009 Starting with "1" = row sums of triangle A178238. - Gary W. Adamson, May 23 2010 The equation n*(n+1)*(n^2 + n + 2)/8 may be arrived at by solving for x in the following equality: (n^2+n)/2 = (sqrt(8x+1)-1)/2. - William A. Tedeschi, Aug 18 2010 Partial sums of A006003. - Jeremy Gardiner, Jun 23 2013 Doubly triangular numbers are revealed in the sums of row sums of Floyd's triangle. 1, 1+5, 1+5+15, ...              1           2     3        4     5     6     7     8     9     10 11    12    13    14    15 - Tony Foster III, Nov 14 2015 From Jaroslav Krizek, Mar 04 2017: (Start) For n>=1; a(n) = sum of the different sums of elements of all the nonempty subsets of the sets of numbers from 1 to n. Example: for n = 6; nonempty subsets of the set of numbers from 1 to 3: {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}; sums of elements of these subsets: 1, 2, 3, 3, 4, 5, 6; different sums of elements of these subsets: 1, 2, 3, 4, 5, 6; a(3) = (1+2+3+4+5+6) = 21, ... (End) a(n) is also the number of 4-cycles in the (n+4)-path complement graph. - Eric W. Weisstein, Apr 11 2018 REFERENCES A. Björner, The homology and shellability of matroids and geometric lattices, in Matroid Applications (ed. N. White), Encyclopedia of Mathematics and Its Applications, 40, Cambridge Univ. Press 1992. L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 124, #25, Q(3,r). N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). R. P. Stanley, Enumerative Combinatorics I, p. 292. LINKS T. D. Noe and William A. Tedeschi, Table of n, a(n) for n=0..10000 (first 1000 terms computed by T. D. Noe) G. E. Andrews, P. Paule, A. Riese and V. Strehl, MacMahon's partition analysis V. Bijections, recursions and magic squares, p. 37. Matthias Beck, The number of "magic" squares and hypercubes, arXiv:math/0201013 [math.CO], 2002-2005. A. G. Bell, Partitioning integers in n dimensions, The Computer Journal, 13 (1970), 278-283. Miklos Bona, A New Proof of the Formula for the Number of 3 × 3 Magic Squares, Mathematics Magazine, Vol. 70, No. 3 (Jun., 1997), pp. 201-203. L. Carlitz, Enumeration of symmetric arrays, Duke Math. J., Vol. 33(4) (1966), pp. 771-782. Brian Conrey and Alex Gamburd, Pseudomoments of the Riemann zeta-function and pseudomagic squares, Journal of Number Theory, Volume 117, Issue 2, April 2006, Pages 263-278. See H4 on p. 269. P. Diaconis and A. Gamburd, Random matrices, magic squares and matching polynomials, Volume 11, Issue 2 (2004-6) (The Stanley Festschrift volume), Research Paper #R2. I. J. Good, On the application of symmetric Dirichlet distributions and their mixtures to contingency tables, Ann. Statist. 4 (1976), no. 6, 1159-1189. I. J. Good, On the application of symmetric Dirichlet distributions and contingency tables, pp. 1178-1179. (Annotated scanned copy) D. M. Jackson and G. H. J. van Rees, The enumeration of generalized double stochastic nonnegative integer square matrices, SIAM J. Comput., 4 (1975), 474-477. D. M. Jackson & G. H. J. van Rees, The enumeration of generalized double stochastic nonnegative integer square matrices, SIAM J. Comput., 4.4 (1975), 474-477. (Annotated scanned copy) Milan Janjic, Two Enumerative Functions Neven Juric, Illustration of the 55 3 X 3 matrices Simon Plouffe, Approximations de séries génératrices et quelques conjectures, Dissertation, Université du Québec à Montréal, 1992. Simon Plouffe, 1031 Generating Functions and Conjectures, Université du Québec à Montréal, 1992. Henry Warburton, On Self-Repeating Series, Transactions of the Cambridge Philosophical Society, Vol. 9, 471-486, 1856. Eric Weisstein's World of Mathematics, Graph Cycle Eric Weisstein's World of Mathematics, Path Complement Graph Index entries for linear recurrences with constant coefficients, signature (5, -10, 10, -5, 1). FORMULA a(n) = 3*binomial(n+2, 4) + binomial(n+1, 2). G.f.: x*(1 + x + x^2)/(1-x)^5. - Simon Plouffe (in his 1992 dissertation) a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4) + 3. - Warut Roonguthai, Dec 13 1999 a(n) = 5a(n-1) - 10a(n-2) + 10a(n-3) - 5a(n-4) + a(n-5) = A000217(A000217(n)). - Ant King, Nov 18 2010 a(n) = Sum(Sum(1 + Sum(3*n))). - Xavier Acloque, Jan 21 2003 a(n) = A000332(n+1) + A000332(n+2) + A000332(n+3), with A000332(n) = binomial(n, 4). - Mitch Harris, Oct 17 2006 and Bruce J. Nicholson, Oct 22 2017 a(n) = Sum_{i=1..C(n,2)} i = C(C(n,2) + 1, 2) = A000217(A000217(n+1)). - Enrique Pérez Herrero, Jun 11 2012 Euler transform of length 3 sequence [6, 0, -1]. - Michael Somos, Nov 19 2015 E.g.f.: x*(8 + 16*x + 8*x^2 + x^3)*exp(x)/8. - Ilya Gutkovskiy, Apr 26 2016 Sum_{n>=1} 1/a(n) = 6 - 4*Pi*tanh(sqrt(7)*Pi/2)/sqrt(7) = 1.25269064911978447... . - Vaclav Kotesovec, Apr 27 2016 a(n) = (n^4 - 2*n^3 + 3*n^2 + 2*n)/8; a(n) = ((n-1)^4 + 3*(n-1)^3 + 2*(n-1)^2 + 2*n))/8. - Bruce J. Nicholson, Apr 05 2017 a(n) = (A016754(n)+ A007204(n)- 2) / 32. - Bruce J. Nicholson, Apr 14 2017 a(n) = a(-1-n) for all n in Z. - Michael Somos, Apr 17 2017 EXAMPLE G.f. = x + 6*x^2 + 21*x^3 + 55*x^4 + 120*x^5 + 231*x^6 + 406*x^7 + 666*x^8 + ... MAPLE A002817 := n->n*(n+1)*(n^2+n+2)/8; MATHEMATICA a[ n_] := n (n + 1) (n^2 + n + 2) / 8; (* Michael Somos, Jul 24 2002 *) LinearRecurrence[{5, -10, 10, -5, 1}, {0, 1, 6, 21, 55}, 40] (* Harvey P. Dale, Jul 18 2011 *) nn=50; Join[{0}, With[{c=(n(n+1))/2}, Flatten[Table[Take[Accumulate[Range[ (nn(nn+1))/2]], {c, c}], {n, nn}]]]] (* Harvey P. Dale, Mar 19 2013 *) PROG (PARI) {a(n) = n * (n+1) * (n^2 + n + 2) / 8}; /* Michael Somos, Jul 24 2002 */ (PARI) concat(0, Vec(x*(1+x+x^2)/(1-x)^5 + O(x^50))) \\ Altug Alkan, Nov 15 2015 CROSSREFS Cf. A000217, A001496, A001621, A064322, A066370, A178238, A005045, A002721. Cf. A006528 (square colorings). Cf. A236770,(see crossrefs). Cf. A016754, A007204 Row n=3 of A257493. Cf. A000332 Cf. A000292 (3-cycle count of \bar P_{n+4}), A060446 (5-cycle count of \bar P_{n+3}), A302695 (6-cycle count of \bar P_{n+5}) Sequence in context: A238702 A162539 A259474 * A132366 A015641 A292950 Adjacent sequences:  A002814 A002815 A002816 * A002818 A002819 A002820 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS More terms from Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 29 1999 Plouffe Maple line edited by N. J. A. Sloane, May 13 2008 STATUS approved

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Last modified November 17 14:18 EST 2018. Contains 317276 sequences. (Running on oeis4.)