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 A088556 Numbers of the form (4^n + 4^(n-1) + ... + 1) + (n mod 2). 2
 6, 21, 86, 341, 1366, 5461, 21846, 87381, 349526, 1398101, 5592406, 22369621, 89478486, 357913941, 1431655766, 5726623061, 22906492246, 91625968981, 366503875926, 1466015503701, 5864062014806, 23456248059221, 93824992236886, 375299968947541, 1501199875790166 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Colin Barker, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (4,1,-4). FORMULA If n is even, then 4^n + ... + 1 = (4^(n+1) - 1)/3 = (2^(n+1) - 1)(2^n+1) + 1)/3. - R. K. Guy, Nov 17 2003 a(n) = 4*a(n-1) + a(n-2) - 4*a(n-3). -  Colin Barker, Apr 02 2012 G.f.: x*(6-3*x-4*x^2) / ((1-x)*(1+x)*(1-4*x)). -  Colin Barker, Apr 02 2012 MATHEMATICA LinearRecurrence[{4, 1, -4}, {6, 21, 86}, 50] (* Vincenzo Librandi, Jun 14 2015 *) PROG (PARI) trajpolypn(n1) = { for(x1=1, n1, y1 = polypn(4, x1); print1(y1", ") ) } polypn(n, p) = { x=n; if(p%2, y=2, y=1); for(m=1, p, y=y+x^m; ); return(y) } (PARI) Vec(x*(6-3*x-4*x^2)/((1-x)*(1+x)*(1-4*x)) + O(x^30)) \\ Colin Barker, Jun 13 2015 (MAGMA) I:=[6, 21, 86]; [n le 3 select I[n] else 4*Self(n-1)+Self(n-2)-4*Self(n-3): n in [1..30]]; // Vincenzo Librandi, Jun 14 2015 CROSSREFS Sequence in context: A219596 A182251 A191597 * A316105 A137966 A304187 Adjacent sequences:  A088553 A088554 A088555 * A088557 A088558 A088559 KEYWORD nonn,easy AUTHOR Cino Hilliard, Nov 17 2003 STATUS approved

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Last modified October 15 09:22 EDT 2019. Contains 328026 sequences. (Running on oeis4.)