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A136325
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a(n) = 8*a(n-1)-a(n-2) with a(0)=0 and a(1)=3.
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3
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0, 3, 24, 189, 1488, 11715, 92232, 726141, 5716896, 45009027, 354355320, 2789833533, 21964312944, 172924670019, 1361433047208, 10718539707645, 84386884613952, 664376537203971, 5230625413017816, 41180626766938557
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OFFSET
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0,2
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COMMENTS
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Nonnegative integers k such that 15*k^2 + 9 is a square.
From the recurrence we have a(n) = sqrt(15)*((4 + sqrt(15))^n - (4 - sqrt(15))^n)/10.
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LINKS
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FORMULA
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a(n) = (sqrt(3/5)*(-(4-sqrt(15))^n + (4+sqrt(15))^n))/2.
G.f.: 3*x/(x^2-8*x+1). (End)
For n > 0, a(n) is the denominator of the continued fraction [2,3,2,3,...,2,3] with n repetitions of 2,3. For the numerators see A070997. - Greg Dresden, Sep 12 2019
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EXAMPLE
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G.f. = 3*x + 24*x^2 + 189*x^3 + 1488*x^4 + 11715*x^5 + 92232*x^6 + 726141*x^7 + ...
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MATHEMATICA
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Do[If[IntegerQ[Sqrt[3 (3 + 5 x^2)]], Print[{x, Sqrt[3 (3 + 5 x^2)]}]], {x, 0, 2000000}]
LinearRecurrence[{8, -1}, {0, 3}, 30] (* Harvey P. Dale, Aug 18 2014 *)
a[ n_] := 3 ChebyshevU[ n - 1, 4]; (* Michael Somos, Oct 14 2015 *)
a[ n_] := 3/2 ((4 + Sqrt[15])^n - (4 - Sqrt[15])^n) / Sqrt[15] // Simplify; (* Michael Somos, Oct 14 2015 *)
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PROG
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(PARI) {a(n) = subst(poltchebi(n+1) - 4 * poltchebi(n), x, 4) / 5}; /* Michael Somos, Apr 05 2008 */
(PARI) {a(n) = 3 * polchebyshev(n-1, 2, 4)}; /* Michael Somos, Oct 14 2015 */
(PARI) {a(n) = 3 * imag( (4 + quadgen(60))^n )}; /* Michael Somos, Oct 14 2015 */
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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Definition, comments, formulas further corrected by Greg Dresden, Sep 13 2019
Exchanged definition and comment, in order to retain offset 0. - N. J. A. Sloane, Sep 23 2019
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STATUS
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approved
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