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 A033436 a(n) = ceiling( (3*n^2 - 4)/8 ). 17
 0, 0, 1, 3, 6, 9, 13, 18, 24, 30, 37, 45, 54, 63, 73, 84, 96, 108, 121, 135, 150, 165, 181, 198, 216, 234, 253, 273, 294, 315, 337, 360, 384, 408, 433, 459, 486, 513, 541, 570, 600, 630, 661, 693, 726, 759, 793, 828 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Number of edges in 4-partite Turan graph of order n. Apart from the initial term this equals the elliptic troublemaker sequence R_n(1,4) (also sequence R_n(3,4)) in the notation of Stange (see Table 1, p.16). For other elliptic troublemaker sequences R_n(a,b) see the cross references below. - Peter Bala, Aug 08 2013 REFERENCES R. L. Graham et al., Handbook of Combinatorics, Vol. 2, p. 1234. LINKS Ivan Panchenko, Table of n, a(n) for n = 0..10000 K. E. Stange, Integral points on elliptic curves and explicit valuations of division polynomials, arXiv:1108.3051 [math.NT], 2011-2014. Eric Weisstein's World of Mathematics, Turán Graph Wikipedia, Turán graph Index entries for linear recurrences with constant coefficients, signature (2,-1,0,1,-2,1). FORMULA The second differences of the listed terms are periodic with period (1, 1, 1, 0) of length 4, showing that the terms satisfy the recurrence a(n) = 2a(n-1)-a(n-2)+a(n-4)-2a(n-5)+a(n-6). - John W. Layman, Jan 23 2001 a(n) = (1/16) {6n^2 - 5 + (-1)^n + 2(-1)^[n/2] - 2(-1)^[(n-1)/2] }. Therefore a(n) is asymptotic to 3/8*n^2. - Ralf Stephan, Jun 09 2005 a(n) = Sum_{i=0..n} {1/8*[(i mod 4)+((i+1) mod 4)-((i+2) mod 4)+3*((i+3) mod 4)]} + a(n-1) - 1, with a(0)=0. - Paolo P. Lava, Jun 28 2007 O.g.f.: -x^2*(1+x+x^2)/((x+1)*(x^2+1)*(x-1)^3). - R. J. Mathar, Dec 05 2007 a(n) = Sum_{k=0..n} A166486(k)*(n-k). - Reinhard Zumkeller, Nov 30 2009 a(n+1) = n + floor(n^2/3). - Gary Detlefs, Mar 27 2010 a(n) = floor(3*n^2/8). - Peter Bala, Aug 08 2013 a(n) = Sum_{i=1..n} floor(3*i/4). - Wesley Ivan Hurt, Sep 12 2017 MAPLE P:=proc(n) local a, i, k, w; w:=0; for i from 0 by 1 to n do a:=sum('1/8*(((k) mod 4)+((k+1) mod 4)-((k+2) mod 4)+3*((k+3) mod 4))', 'k'=0..i)-1; w:=w+a; print(w); od; end: P(100); # Paolo P. Lava, Jun 28 2007 MATHEMATICA LinearRecurrence[{2, -1, 0, 1, -2, 1}, {0, 0, 1, 3, 6, 9}, 48] (* Jean-François Alcover, Sep 21 2017 *) PROG (PARI) a(n)=(3*n^2 +3)\8 \\ Charles R Greathouse IV, Sep 24 2015 CROSSREFS Cf. A002620 (= R_n(1,2)), A000212 (= R_n(1,3) = R_n(2,3)), A033437 (= R_n(1,5) = R_n(4,5)), A033438 (= R_n(1,6) = R_n(5,6)), A033439 (= R_n(1,7) = R_n(6,7)), A033440, A033441, A033442, A033443, A033444. Cf. A007590 (= R_n(2,4)), A030511 (= R_n(2,6) = R_n(4,6)), A184535 (= R_n(2,5) = R_n(3,5)). Sequence in context: A048202 A014785 A132352 * A059293 A002578 A129728 Adjacent sequences:  A033433 A033434 A033435 * A033437 A033438 A033439 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified October 23 09:28 EDT 2019. Contains 328345 sequences. (Running on oeis4.)