OFFSET
1,1
COMMENTS
Previous name was: a(n) = round(phi^(6^n)) where phi = 1.6180339887498948482... = (sqrt(5)+1)/2.
General (hyperbolic) trigonometric formula for a(n) = round(phi^((2k)^n)) = 2*cosh((2k)^n*arccosh(sqrt(5)/2) where phi = 1.6180339887498948482... = (sqrt(5)+1)/2. [Artur Jasinski, Oct 09 2008]
FORMULA
a(n) = G^(6^n) + (1 - G)^(6^n) = G^(6^n) + (-G)^(-6^n) where G is the golden ratio A001622. [Artur Jasinski, Oct 05 2008]
a(n) = 2*cosh(6^n*arccosh(sqrt(5)/2). [Artur Jasinski, Oct 09 2008]
From Peter Bala, Nov 28 2022: (Start)
a(n) = Lucas(6^n).
a(n+1) = a(n)^6 - 6*a(n)^4 + 9*a(n)^2 - 2 with a(1) = 18. (End)
MAPLE
a := proc(n) option remember; if n = 1 then 18 else a(n-1)^6 - 6*a(n-1)^4 + 9*a(n-1)^2 - 2 end if; end;
seq(a(n), n = 1..5); # Peter Bala, Nov 28 2022
MATHEMATICA
c = N[GoldenRatio, 1000]; Table[Round[c^(6^n)], {n, 1, 5}]
c = (1 + Sqrt[5])/2; Table[Expand[c^(6^n) + (1 - c)^(6^n)], {n, 0, 5}] (* Artur Jasinski, Oct 05 2008 *)
Table[Round[N[2*Cosh[6^n*ArcCosh[Sqrt[5]/2]], 100]], {n, 1, 4}] (* Artur Jasinski, Oct 09 2008 *)
CROSSREFS
KEYWORD
nonn,bref
AUTHOR
Artur Jasinski, Sep 22 2008
EXTENSIONS
New name from Peter Bala, Nov 28 2022
STATUS
approved