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 A080956 a(n) = (n+1)*(2-n)/2. 33
 1, 1, 0, -2, -5, -9, -14, -20, -27, -35, -44, -54, -65, -77, -90, -104, -119, -135, -152, -170, -189, -209, -230, -252, -275, -299, -324, -350, -377, -405, -434, -464, -495, -527, -560, -594, -629, -665, -702, -740, -779, -819, -860, -902, -945, -989, -1034, -1080, -1127, -1175, -1224, -1274, -1325, -1377 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Coefficient of x in the polynomial C(n,0)+C(n+1,1)x+C(n+2,2)x(x-1)/2. Equals A154990 * [1,2,3,...]. - Gary W. Adamson & Mats Granvik, Jan 19 2009 a(n) is essentially the case 1 of the polygonal numbers. The polygonal numbers are defined as P_k(n) = Sum_{i=1..n} ((k-2)*i-(k-3)). Thus P_1(n) = n*(3-n)/2 and a(n) = P_1(n+1). See A005563 for the case k=0. - Peter Luschny, Jul 08 2011 This is the case k=-1 of the formula (k*m*(m+1)-(-1)^k+1)/2. See similar sequences listed in A262221. - Bruno Berselli, Sep 17 2015 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..10000 Paul Barry, Centered polygon numbers, heptagons and nonagons, and the Robbins numbers, arXiv:2104.01644 [math.CO], 2021. Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = 2*(C(n+1, 1)-C(n+2, 2)) = (n+1)*(2-n)/2. G.f.: (1-2*x)/(1-x)^3. - R. J. Mathar, Jun 11 2009 If we define f(n,i,a) = Sum_{k=0..n-i} (binomial(n,k)*stirling1(n-k,i)*Product_{j=0..k-1} (-a-j)), then a(n) = f(n,n-1,2), for n>=3. - Milan Janjic, Dec 20 2008 E.g.f.: exp(x)*(1-x^2/2). - Zerinvary Lajos, Apr 05 2009, R. J. Mathar, Jun 11 2009 a(n) = - A214292(n,1) for n > 0. - Reinhard Zumkeller, Jul 12 2012 Recurrence: a(0)=1, a(n+1) = a(n) - n. Also a(n)=(n+1)-Sum[k=1..n](k). Also a(n) = A000027(n+1) - A000217(n). Also, for n>1, a(n) = - A000096(n-2). - Stanislav Sykora, Feb 19 2014 Sum_{n>=3} 1/a(n) = -11/9. - Amiram Eldar, Sep 26 2022 EXAMPLE a(5) = 6-(1+2+3+4+5). - Stanislav Sykora, Feb 19 2014 MAPLE G(x):=exp(x)*(x-x^2/2): f[0]:=G(x): for n from 1 to 54 do f[n]:=diff(f[n-1], x) od: x:=0: seq(f[n], n=1..54 ); # Zerinvary Lajos, Apr 05 2009 MATHEMATICA f[n_] := n; lst = {}; Do[a = f[n]; Do[a -= f[m], {m, n - 1, 1, -1}]; AppendTo[lst, a], {n, 46}]; lst (* Vladimir Joseph Stephan Orlovsky, Feb 10 2010 *) FoldList[#1 - #2 &, 1, Range[0, 44]] (* Arkadiusz Wesolowski, May 26 2013 *) LinearRecurrence[{3, -3, 1}, {1, 1, 0}, 60] (* Harvey P. Dale, Nov 29 2019 *) PROG (Magma) [(n+1)*(2-n)/2: n in [0..80]]; // Vincenzo Librandi, Jul 08 2011 (PARI) a(n)=(n+1)*(2-n)/2; CROSSREFS Cf. A000027, A000096, A000217, A154990, A214292, A262221. Sequence in context: A132336 A272370 A212342 * A132337 A000096 A134189 Adjacent sequences: A080953 A080954 A080955 * A080957 A080958 A080959 KEYWORD sign,easy AUTHOR Paul Barry, Mar 01 2003 EXTENSIONS Lajos e.g.f. adapted to offset zero by R. J. Mathar, Jun 11 2009 STATUS approved

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Last modified December 6 14:43 EST 2022. Contains 358644 sequences. (Running on oeis4.)