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A146509
Numbers that are congruent to {1, 5} mod 18.
5
1, 5, 19, 23, 37, 41, 55, 59, 73, 77, 91, 95, 109, 113, 127, 131, 145, 149, 163, 167, 181, 185, 199, 203, 217, 221, 235, 239, 253, 257, 271, 275, 289, 293, 307, 311, 325, 329, 343, 347, 361, 365, 379, 383, 397, 401, 415, 419, 433, 437, 451, 455, 469, 473, 487
OFFSET
1,2
COMMENTS
Positive integers k such that Hypergeometric[k/6,(6-k)/6,1/2,3/4] = 2Cos[2Pi/9].
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