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 A057570 Numbers of the form n*(7n+-1)/2. 18
 0, 3, 4, 13, 15, 30, 33, 54, 58, 85, 90, 123, 129, 168, 175, 220, 228, 279, 288, 345, 355, 418, 429, 498, 510, 585, 598, 679, 693, 780, 795, 888, 904, 1003, 1020, 1125, 1143, 1254, 1273, 1390, 1410, 1533, 1554, 1683, 1705, 1840, 1863, 2004 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also integers of the form sum(k = 1..n, k/7). - Alonso del Arte, Jan 20 2012 Sequence provides all integers m such that 56*m + 1 is a square. [Bruno Berselli, Oct 07 2015] LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (1,2,-2,-1,1). FORMULA G.f. -x^2*(3+x+3*x^2) / ( (1+x)^2*(x-1)^3 ). - R. J. Mathar, Jan 25 2011 a(n) = +1*a(n-1)+2*a(n-2)-2*a(n-3)-1*a(n-4)+1*a(n-5). - Joerg Arndt, Jan 25 2011 a(n) = (14*n*(n-1)+5*(2*n-1)*(-1)^n+5)/16. - Bruno Berselli, Jan 25 2011 a(n)-a(n-2) = A047341(n-1) for n>2. - Bruno Berselli, Jan 25 2011 Sum_{n>=2} 1/a(n) = 14 - 2*cot(Pi/7)*Pi. - Amiram Eldar, Mar 17 2022 MATHEMATICA lst={}; s=0; Do[s+=n/7; If[Floor[s]==s, AppendTo[lst, s]], {n, 0, 7!}]; lst (* Vladimir Joseph Stephan Orlovsky, Jan 06 2009 *) Select[Table[Plus@@Range[n]/7, {n, 0, 199}], IntegerQ] (* Alonso del Arte, Jan 20 2012 *) CoefficientList[Series[-x (3 + x + 3 x^2) / ((1 + x)^2 (x - 1)^3), {x, 0, 40}], x] (* Vincenzo Librandi, Jun 19 2013 *) PROG (PARI) a(n)=(14*n*(n-1)+5*(2*n-1)*(-1)^n+5)/16 \\ Charles R Greathouse IV, Sep 24 2015 CROSSREFS Cf. A074378, A001318, A057569, A154260, A154292, A154293, A047341. Sequence in context: A278426 A293280 A087884 * A024853 A023857 A323149 Adjacent sequences: A057567 A057568 A057569 * A057571 A057572 A057573 KEYWORD nonn,easy AUTHOR N. J. A. Sloane, Oct 04 2000 STATUS approved

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Last modified November 26 21:15 EST 2022. Contains 358362 sequences. (Running on oeis4.)