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A360467
a(n) = Fibonacci(4*n+2) + 3*Fibonacci(2*n+1)^2.
2
4, 20, 130, 884, 6052, 41474, 284260, 1948340, 13354114, 91530452, 627359044, 4299982850, 29472520900, 202007663444, 1384581123202, 9490060198964, 65045840269540, 445830821687810, 3055769911545124, 20944558559128052, 143556140002351234, 983948421457330580
OFFSET
0,1
COMMENTS
Values of x + 3*y in solutions of x^2 = 5*y^2 - 4*y in positive integers. In the solutions, the values of x and y are given by Fibonacci(4*n + 2) and Fibonacci(2*n + 1)^2 respectively.
The above Diophantine equation arises out of the following problem regarding the subdivision of a square into four triangles of integer area. For n >= 1, the sequence gives the areas of the squares in the solutions (see illustration in Links). Two lines are drawn from a corner of a square to points on the opposing sides. A third line is added between the two points so that the square is divided into four triangles. The area of each triangle is required to be an integer and those of the right triangles to form an arithmetic progression with difference 1. The smallest right triangle by area is the one formed by the third line. In the solutions, the area of the inner triangle is given by Fibonacci(4*n + 2) and the total area of the three right triangles is 3*Fibonacci(2*n + 1)^2. The area of the square is then equal to a(n).
FORMULA
a(n) = A033890(n) + 3*A081068(n)^2.
a(n) = Fibonacci(2*n+1)*(Fibonacci(2*n) + Fibonacci(2*n+2) + 3*Fibonacci(2*n+1)).
a(n) = 2*A064170(n+3).
G.f.: 2*(2 - 6*x + x^2)/((1 - x)*(1 - 7*x + x^2)). - Andrew Howroyd, Feb 16 2023
a(n) = a(n-3) - 8 * (a(n-2) - a(n-1)) for n >= 3. - Peter Luschny, Feb 17 2023
a(n) = a(-2-n) = 2*F{2*n+1) * F(2*n+3) = A295683(4*(n+1)) for all n in Z. - Michael Somos, Mar 02 2023
EXAMPLE
a(2) = F(4*2+2) + 3*F(2*2 +1)^2 = F(10) + 3*F(5)^2 = 55 + 3*5^2 = 130.
a(4) = F(4*4+2) + 3*F(2*4 +1)^2 = F(18) + 3*F(9)^2 = 2584+ 3*34^2 = 6052.
G.f. = 4 + 20*x + 130*x^2 + 884*x^3 + 6052*x^4 + ... - Michael Somos, Mar 02 2023
MAPLE
a := proc(n) option remember; if n < 3 then return [4, 20, 130][n + 1] fi;
a(n-3) - 8 * (a(n-2) - a(n-1)) end: seq(a(n), n = 0..22); # Peter Luschny, Feb 17 2023
MATHEMATICA
LinearRecurrence[{8, -8, 1}, {4, 20, 130}, 22] (* Amiram Eldar, Feb 17 2023 *)
a[ n_] := 2 * Fibonacci[2*n+1] * Fibonacci[2*n+3]; (* Michael Somos, Mar 02 2023 *)
PROG
(PARI) Vec(2*(2 - 6*x + x^2)/((1 - x)*(1 - 7*x + x^2)) + O(x^25)) \\ Andrew Howroyd, Feb 16 2023
(SageMath)
print([2*(lucas_number2(n+1, 7, 1) + 3) // 5 for n in range(23)]) # Peter Luschny, Feb 17 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved