OFFSET
0,2
COMMENTS
Cell configuration converges to a fractal carpet with dimension 1.818...
LINKS
Colin Barker, Table of n, a(n) for n = 0..1000
Peter Karpov, InvMem, Item 26
Peter Karpov, Illustration of cell configuration after 5 iterations
Index entries for linear recurrences with constant coefficients, signature (9,-12).
FORMULA
a(0) = 1, a(1) = 5, a(2) = 36, a(n) = 9*a(n-1) - 12*a(n-2).
G.f.: (1-4*x+3*x^2)/(1-9*x+12*x^2).
a(n) = (2^(-3-n)*((9-sqrt(33))^n*(-13+3*sqrt(33)) + (9+sqrt(33))^n*(13+3*sqrt(33)))) / sqrt(33) for n>0. - Colin Barker, Apr 18 2017
a(n) = (1/4)*([n=0] + (2*sqrt(3))^(n-1)*( 6*sqrt(3)*ChebyshevU(n, 9/(4*sqrt(3))) - 7*ChebyshevU(n-1, 9/(4*sqrt(3))) ) ). - G. C. Greubel, Dec 11 2021
MATHEMATICA
{1}~Join~LinearRecurrence[{9, -12}, {5, 36}, 16]
PROG
(PARI) Vec((1 - x)*(1 - 3*x) / (1 - 9*x + 12*x^2) + O(x^30)) \\ Colin Barker, Apr 18 2017
(Magma) I:=[5, 36]; [1] cat [n le 2 select I[n] else 9*Self(n-1) - 12*Self(n-2): n in [1..31]]; // G. C. Greubel, Dec 11 2021
(Sage) [(1/4)*(bool(n==0) + (2*sqrt(3))^(n-1)*( 6*sqrt(3)*chebyshev_U(n, 9/(4*sqrt(3))) - 7*chebyshev_U(n-1, 9/(4*sqrt(3))) ) ) for n in (0..30)] # G. C. Greubel, Dec 11 2021
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Peter Karpov, Apr 18 2017
EXTENSIONS
More terms from Colin Barker, Apr 18 2017
STATUS
approved