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 A156084 Sum of the squares of the first n Fibonacci numbers with index divisible by 3. 4
 0, 4, 68, 1224, 21960, 394060, 7071116, 126886032, 2276877456, 40856908180, 733147469780, 13155797547864, 236071208391768, 4236125953503964, 76014195954679580, 1364019401230728480, 24476335026198433056, 439210011070341066532 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Natural bilateral extension (brackets mark index 0): ..., -21960, -1224, -68, -4, 0, [0], 4, 68, 1224, 21960, 394060, ... This is (-A156084)-reversed followed by A156084. That is, A156084(-n) = -A156084(n-1). LINKS Index entries for linear recurrences with constant coefficients, signature (17,17,-1). FORMULA Let F(n) be the Fibonacci number A000045(n) and let L(n) be the Lucas number A000032(n). a(n) = sum_{k=1..n} F(3k)^2. Closed form: a(n) = L(6n+3)/20 - (-1)^n/5. Factored closed form: a(n) = (1/4) F(n) F(n+1) (L(n) - 1)(L(n) + 1)(L(2n+2) - 1) if n is even; a(n) = (1/4) F(n) F(n+1) (L(n+1) - 1)(L(n+1) + 1)(L(2n) - 1) if n is odd. Recurrence: a(n) - 17 a(n-1) - 17 a(n-2) + a(n-3) = 0. G.f.: A(x) = 4 x/(1 - 17 x - 17 x^2 + x^3) = 4 x/((1 + x)(1 - 18 x + x^2)). a(n) = 4*A156085(n). - R. J. Mathar, Aug 06 2019 MATHEMATICA a[n_Integer] := If[ n >= 0, Sum[ Fibonacci[3k]^2, {k, 1, n} ], -Sum[ Fibonacci[-3k]^2, {k, 1, -n - 1} ] ] CROSSREFS Partial sums of A014729. Cf. A156085, A156090, A156091 Sequence in context: A339787 A323276 A141032 * A000658 A156470 A326288 Adjacent sequences:  A156081 A156082 A156083 * A156085 A156086 A156087 KEYWORD nonn,easy AUTHOR Stuart Clary, Feb 04 2009 STATUS approved

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Last modified July 26 20:56 EDT 2021. Contains 346300 sequences. (Running on oeis4.)