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 A242658 a(n) = 3n^2-3n+2. 3
 2, 2, 8, 20, 38, 62, 92, 128, 170, 218, 272, 332, 398, 470, 548, 632, 722, 818, 920, 1028, 1142, 1262, 1388, 1520, 1658, 1802, 1952, 2108, 2270, 2438, 2612, 2792, 2978, 3170, 3368, 3572, 3782, 3998, 4220, 4448, 4682, 4922, 5168, 5420, 5678, 5942, 6212, 6488, 6770, 7058, 7352 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS An exercise in Smith (1950), my secondary school algebra book. For n > 0, also the number of (not necessarily maximum) cliques in the (n-1)-triangular grid graph. - Eric W. Weisstein, Nov 29 2017 REFERENCES C. Smith, A Treatise on Algebra, Macmillan, London, 5th ed., 1950, p. 429, Example 2(i). LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Eric Weisstein's World of Mathematics, Clique Eric Weisstein's World of Mathematics, Triangular Grid Graph Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA From Chai Wah Wu, May 30 2016: (Start) a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n > 2. G.f.: 2*(-4*x^2 + 2*x - 1)/(x - 1)^3. (End) MATHEMATICA Table[3 n^2 - 3 n + 2, {n, 0, 100}] (* Vincenzo Librandi, Sep 05 2016 *) LinearRecurrence[{3, -3, 1}, {2, 8, 20}, {0, 20}] (* Eric W. Weisstein, Nov 29 2017 *) CoefficientList[Series[-2 (1 - 2 x + 4 x^2)/(-1 + x)^3, {x, 0, 20}], x] (* Eric W. Weisstein, Nov 29 2017 *) PROG (MAGMA) [3*n^2 - 3*n + 2: n in [0..70]]; // Vincenzo Librandi, Sep 05 2016 (PARI) a(n) = 3*n^2-3*n+2 \\ Altug Alkan, Sep 05 2016 CROSSREFS A077588 is the same except for the initial term. Cf. A242659. Sequence in context: A208966 A067640 A098277 * A080040 A060823 A178076 Adjacent sequences:  A242655 A242656 A242657 * A242659 A242660 A242661 KEYWORD nonn,easy AUTHOR N. J. A. Sloane, May 30 2014 STATUS approved

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Last modified December 14 14:50 EST 2019. Contains 329979 sequences. (Running on oeis4.)