OFFSET
1,1
COMMENTS
For any x=f(1) positive integer>1 there is an integer N(x) such that for any n>=N(x) f(n)/f(n-3)=4. N(2)=6 N(3)=5 N(4)=5 ... N(1479) =20. Conjecture: N(x) is asymptotic to C*Log(x) with C=0.3..... ( 3/10 < C < 4/10).
LINKS
Robert Israel, Table of n, a(n) for n = 1..4978
Index entries for linear recurrences with constant coefficients, signature (0, 0, 4).
FORMULA
a(1)=2, a(2)=3, a(3)=4, a(4)=6, a(5)=10 and for n> 5 a(n) = 4*a(n-3).
G.f.: (1+x)/2 + (3 + 5*x + 8*x^2)/(2*(1-4*x^3)). - Robert Israel, Dec 09 2019
MAPLE
2, 3, seq(op([2^(2*n), 3*2^(2*n-1), 5*2^(2*n-1)]), n=1..20); # Robert Israel, Dec 09 2019
MATHEMATICA
NestList[2*#-EulerPhi[#]&, 2, 40] (* Harvey P. Dale, Oct 30 2013 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Aug 14 2002
STATUS
approved