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 A086341 a(n) = 2*2^floor(n/2) - (-1)^n. 5
 1, 3, 3, 5, 7, 9, 15, 17, 31, 33, 63, 65, 127, 129, 255, 257, 511, 513, 1023, 1025, 2047, 2049, 4095, 4097, 8191, 8193, 16383, 16385, 32767, 32769, 65535, 65537, 131071, 131073, 262143, 262145, 524287, 524289, 1048575, 1048577, 2097151, 2097153 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..2000 Index entries for linear recurrences with constant coefficients, signature (-1,2,2). FORMULA E.g.f.: 2*cosh(sqrt(2)*x) + 2*sinh(sqrt(2)*x)/sqrt(2) - sinh(x) + cosh(x). a(n) = (1 + 1/sqrt(2))*sqrt(2)^n + (1 - 1/sqrt(2))*(-sqrt(2))^n - (-1)^n. G.f.: (1+2*x)^2/((1+x)*(1-2*x^2)). - Colin Barker, Aug 17 2012 a(n) = a(n-1) + 2*a(n-2) + 2*a(n-3); a(0)=1, a(1)=3, a(2)=3. - Harvey P. Dale, Mar 10 2013 From Amiram Eldar, Sep 14 2022: (Start) Sum_{n>=0} 1/a(n) = A065442 + A323482 - 1/2. Sum_{n>=0} (-1)^n/a(n) = 2 * A248721. (End) MATHEMATICA CoefficientList[Series[(1+2x)^2/((1+x)(1-2x^2)), {x, 0, 50}], x] (* or *) LinearRecurrence[ {-1, 2, 2}, {1, 3, 3}, 50] (* Harvey P. Dale, Mar 10 2013 *) PROG (Magma) [2*2^Floor(n/2)-(-1)^n: n in [0..40]]; // Vincenzo Librandi, Aug 16 2011 (PARI) vector(40, n, n--; 2^(floor(n/2)+1) - (-1)^n) \\ G. C. Greubel, Nov 08 2018 CROSSREFS Cf. A016116 (2^floor(n/2)). Cf. A065442, A248721, A323482. Sequence in context: A323430 A291941 A355225 * A176513 A128424 A129758 Adjacent sequences: A086338 A086339 A086340 * A086342 A086343 A086344 KEYWORD easy,nonn AUTHOR Paul Barry, Jul 16 2003 STATUS approved

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Last modified February 27 15:42 EST 2024. Contains 370377 sequences. (Running on oeis4.)