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 A084175 Jacobsthal oblong numbers. 13
 0, 1, 3, 15, 55, 231, 903, 3655, 14535, 58311, 232903, 932295, 3727815, 14913991, 59650503, 238612935, 954429895, 3817763271, 15270965703, 61084037575, 244335800775, 977343902151, 3909374210503, 15637499638215, 62549992960455 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Inverse binomial transform is A001019 doubled up. Binomial transform is A084177. Partial sums of A003683. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..500 N. Heninger, E. M. Rains and N. J. A. Sloane, On the Integrality of n-th Roots of Generating Functions, arXiv:math/0509316 [math.NT], 2005-2006. N. Heninger, E. M. Rains and N. J. A. Sloane, On the Integrality of n-th Roots of Generating Functions, J. Combinatorial Theory, Series A, 113 (2006), 1732-1745. Index entries for linear recurrences with constant coefficients, signature (3,6,-8). FORMULA a(n) = A001045(n)*A001045(n+1). a(n) = (2*4^n - (-2)^n - 1)/9; a(n) = 3*a(n-1) + 6*a(n-2) - 8*a(n-3), a(0)=0, a(1)=1, a(2)=3. G.f.: x/((1+2*x)*(1-x)*(1-4*x)). E.g.f.: (2*exp(4*x) - exp(x) - exp(-2*x))/9. a(n+1) - 4*a(n) = 1, -1, 3, -5, 11, ... = A001045(n+1) signed. - Paul Curtz, May 19 2008 a(n) = round(2^n/3) * round(2^(n+1)/3). - Gary Detlefs, Feb 10 2010 From Peter Bala, Mar 30 2015: (Start) The shifted o.g.f. A(x) := 1/( (1 + 2*x)*(1 - x)*(1 - 4*x) ) = 1/(1 - 3*x - 6*x^2 + 8*x^3). Hence A(x) == 1/(1 - 3*x + 3*x^2 - x^3) (mod 9) == 1/(1 - x)^3 (mod 9). It follows by Theorem 1 of Heninger et al. that (A(x))^(1/3) = 1 + x + 4*x^2 + 10*x^3 + ... has integral coefficients. Sum_{n >= 0} a(n+1)*x^n = exp( Sum_{n >= 1} J(3*n)/J(n)*x^n/n ), where J(n) = A001045(n) are the Jacobsthal numbers. Cf. A001656, A099930. (End) MAPLE for n from 1 to 25 do print(round(2^n/3)*round(2^(n+1)/3)) od; # Gary Detlefs, Feb 10 2010 MATHEMATICA Table[(2*4^n -(-2)^n -1)/9, {n, 0, 30}] (* Vladimir Joseph Stephan Orlovsky, Feb 05 2011, modified by G. C. Greubel, Sep 21 2019 *) LinearRecurrence[{3, 6, -8}, {0, 1, 3}, 25] (* Jean-François Alcover, Sep 21 2017 *) PROG (Sage) [gaussian_binomial(n, 2, -2) for n in range(1, 26)] # Zerinvary Lajos, May 28 2009 (MAGMA) [(2*4^n-(-2)^n-1)/9: n in [0..30]]; // Vincenzo Librandi, Jun 04 2011 (PARI) a(n)=(2*4^n-(-2)^n-1)/9 \\ Charles R Greathouse IV, Sep 24 2015 (GAP) List([0..30], n-> (2^(2*n+1) -(-2)^n -1)/9); # G. C. Greubel, Sep 21 2019 CROSSREFS Except for initial terms, same as A015249 and A084152. Cf. A001654, A001656, A084158, A084159, A099930. Sequence in context: A261737 A015249 A084152 * A081951 A033853 A284014 Adjacent sequences:  A084172 A084173 A084174 * A084176 A084177 A084178 KEYWORD easy,nonn AUTHOR Paul Barry, May 18 2003 STATUS approved

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Last modified February 19 00:35 EST 2020. Contains 332028 sequences. (Running on oeis4.)