OFFSET
1,2
COMMENTS
Positive integers k such that Hypergeometric[k/6,(6-k)/6,1/2,3/4] = 2Cos[2Pi/9].
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (1,1,-1).
FORMULA
a(2k-1) = 18*(k-1)+1, a(2k) = 18*(k-1)+5, where k>0.
G.f.: x*(1+4*x+13*x^2)/((1+x)*(1-x)^2). - Vincenzo Librandi, Jul 11 2012
a(n) = (18*n - 5*(-1)^n - 21)/2. - Bruno Berselli, Jul 12 2012 [Corrected by David Lovler, Sep 24 2022]
a(1)=1, a(n) = 18*n -a(n-1) -30. - Vincenzo Librandi, Jul 12 2012
E.g.f.: 13 + ((18*x - 21)*exp(x) - 5*exp(-x))/2. - David Lovler, Sep 05 2022
MATHEMATICA
Select[Range[500], MemberQ[{1, 5}, Mod[#, 18]]&] (* Harvey P. Dale, Jul 24 2011 *)
PROG
(Magma) [n: n in [1..500] | n mod 18 in [1, 5]]; // Bruno Berselli, Jul 12 2012
(PARI) a(n)=n\2*18+if(n%2, 1, -13) \\ Charles R Greathouse IV, Jul 14 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Artur Jasinski, Oct 30 2008
EXTENSIONS
Crossrefs corrected by Ray Chandler, Dec 06 2016
STATUS
approved