OFFSET
1,2
FORMULA
Conjectures from Colin Barker, Apr 28 2018: (Start)
G.f.: x*(1 + x + x^2) / ((1 - x)*(1 - 3*x - 3*x^2 - x^3)).
a(n) = 4*a(n-1) - 2*a(n-3) - a(n-4) for n>4.
(End)
MAPLE
a := proc(n) option remember;
if n<1 then 0 else
if n=1 then 1 else ceil(a(n-1)/(2^(1/3)-1)+1)
end if end if end proc;
seq(a(n), n=0..10);
MATHEMATICA
Nest[Append[#, Ceiling[#[[-1]]/(2^(1/3) - 1) + 1]] &, {1}, 25] (* or *)
Rest@ CoefficientList[Series[x (1 + x + x^2)/((1 - x) (1 - 3 x - 3 x^2 - x^3)), {x, 0, 25}], x] (* Michael De Vlieger, Apr 28 2018 *)
PROG
(PARI) a(n) = if (n==1, 1, ceil(a(n-1)/(2^(1/3)-1)+1)); \\ Michel Marcus, Apr 28 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Gregory Gerard Wojnar, Apr 27 2018
STATUS
approved