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 A084159 Pell oblongs. 14
 1, 3, 21, 119, 697, 4059, 23661, 137903, 803761, 4684659, 27304197, 159140519, 927538921, 5406093003, 31509019101, 183648021599, 1070379110497, 6238626641379, 36361380737781, 211929657785303, 1235216565974041 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Essentially the same as A046727. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 P. E. Trier, "Almost Isosceles" Right-Angled Triangles, Eureka, No. 4, May 1940, pp. 9 - 11. Index entries for linear recurrences with constant coefficients, signature (5,5,-1). FORMULA a(n) = ((sqrt(2)+1)^(2*n+1) - (sqrt(2)-1)^(2*n+1) + 2*(-1)^n)/4. a(n) = 5*a(n-1) + 5*a(n-2) - a(n-3). - Paul Curtz, May 17 2008 G.f.: (1-x)^2/((1+x)*(1-6*x+x^2)). - R. J. Mathar, Sep 17 2008 a(n) = A078057(n)*A001333(n). - R. J. Mathar, Jul 08 2009 a(n) = A001333(n)*A001333(n+1). From Peter Bala, May 01 2012: (Start) a(n) = (-1)^n*R(n,-4), where R(n,x) is the n-th row polynomial of A211955. a(n) = (-1)^n*1/u*T(n,u)*T(n+1,u) with u = sqrt(-1) and T(n,x) the Chebyshev polynomial of the first kind. a(n) = (-1)^n + 4*Sum_{k = 1..n} (-1)^(n-k)*8^(k-1)*binomial(n+k,2*k). Recurrence equations: a(n) = 6*a(n-1) - a(n-2) + 4*(-1)^n, with a(0) = 1 and a(1) = 3; a(n)*a(n-2) = a(n-1)*(a(n-1)+4*(-1)^n). Sum_{k >= 0} (-1)^k/a(k) = 1/sqrt(2). 1 - 2*(Sum_{k = 0..n} (-1)^k/a(k))^2 = (-1)^(n+1)/A090390(n+1). (End) a(n) = (A001333(2*n+1) + (-1)^n)/2. - G. C. Greubel, Oct 11 2022 MATHEMATICA b[n_]:= Numerator[FromContinuedFraction[ContinuedFraction[Sqrt[2], n]]]; Join[{1}, Table[b[n+1], {n, 50}]*Table[b[n], {n, 50}]] (* Vladimir Joseph Stephan Orlovsky, Jan 15 2011 *) LinearRecurrence[{5, 5, -1}, {1, 3, 21}, 30] (* Harvey P. Dale, Aug 04 2019 *) PROG (Magma) [Floor(((Sqrt(2)+1)^(2*n+1)-(Sqrt(2)-1)^(2*n+1)+2*(-1)^n)/4): n in [0..30]]; // Vincenzo Librandi, Aug 13 2011 (SageMath) [(lucas_number2(2*n+1, 2, -1) + 2*(-1)^n)/4 for n in range(31)] # G. C. Greubel, Oct 11 2022 CROSSREFS Cf. A046727 (same sequence except for first term). Cf. A001333, A001654, A078057, A084158, A084175, A090390, A182432, A211955. Sequence in context: A005057 A092634 A178537 * A046727 A283421 A117512 Adjacent sequences: A084156 A084157 A084158 * A084160 A084161 A084162 KEYWORD easy,nonn AUTHOR Paul Barry, May 18 2003 STATUS approved

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Last modified March 28 03:48 EDT 2023. Contains 361577 sequences. (Running on oeis4.)