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 A099721 a(n) = n^2*(2*n+1). 8
 0, 3, 20, 63, 144, 275, 468, 735, 1088, 1539, 2100, 2783, 3600, 4563, 5684, 6975, 8448, 10115, 11988, 14079, 16400, 18963, 21780, 24863, 28224, 31875, 35828, 40095, 44688, 49619, 54900, 60543, 66560, 72963, 79764, 86975, 94608, 102675, 111188 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS For a right triangle with sides of lengths 8*n^3 + 12*n^2 + 8*n + 2, 4*n^4 + 8*n^3 + 4*n^2, and 4*n^4 + 8*n^3 + 12*n^2 + 8*n + 2, dividing the area by the perimeter gives a(n). - J. M. Bergot, Jul 30 2013 This sequence is the difference between the centered icosahedral (or cuboctahedral) numbers (A005902(n)) and the centered octagonal pyramidal numbers (A000447(n+1)). - Peter M. Chema, Jan 09 2016 a(n) is the sum of the integers in the closed interval (n-1)*n to n*(n+1). - J. M. Bergot, Apr 19 2017 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Index entries for linear recurrences with constant coefficients, signature (4,-6,4,-1). FORMULA G.f.: x*(3 + 8*x + x^2)/(x-1)^4. a(n) = A024196(n) - A024196(n-1). - Philippe Deléham, May 07 2012 a(n) = ceiling(Sum_{i=n^2-(n-1)..n^2+(n-1)} s(i)), for n > 0 and integer i, where s(i) are the real solutions to x = i + sqrt(x), and the summation range excludes the integer solutions which occur where i is an oblong number (A002378). The fractional portion of the summation converges to 2/3 for large n. If s(i) is replaced with i, then the summation equals n^2*(2*n-1) = A015237. - Richard R. Forberg, Oct 15 2014 a(n) = A005902(n) - A000447(n+1). - Peter M. Chema, Jan 09 2016 MAPLE A099721 := proc(n) n^2*(2*n+1) ; end proc: seq(A099721(n), n=0..10) ; MATHEMATICA a[n_]:=2*n^3+n^2; ...and/or...Array[ #*(#*(2*#+1))&, 5!, 0] (* Vladimir Joseph Stephan Orlovsky, Dec 21 2008 *) PROG (MAGMA) [n^2*(2*n+1): n in [0..50]]; // Vincenzo Librandi, May 01 2011 (PARI) a(n) = ceil(sum(i=n^2-(n-1), n^2+(n-1), if(!issquare(4*i+1), (2*i+1+sqrt(4*i+1))/2, 0))); \\ Michel Marcus, Nov 14 2014, after Richard R. Forberg CROSSREFS Cf. A000578, A066023, A001093, A034262, A071568, A011379, A027444, A053698, A033431, A033562, A061317, A098547, A015237. Row n=3 of A229079. Sequence in context: A143582 A132404 A062359 * A024402 A292072 A183377 Adjacent sequences:  A099718 A099719 A099720 * A099722 A099723 A099724 KEYWORD nonn,easy AUTHOR Douglas Winston (douglas.winston(AT)srupc.com), Nov 07 2004 STATUS approved

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Last modified April 7 04:20 EDT 2020. Contains 333292 sequences. (Running on oeis4.)