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A251558 a(n) = smallest odd number not in {A098550(1), A098550(2), ..., A098550(n)} which is neither a prime nor a term of A251542. 4
9, 9, 9, 9, 15, 15, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 27, 27, 33, 33, 33, 33, 33, 45, 45, 49, 49, 49, 49, 49, 49, 49, 49, 49, 49, 49, 49, 49, 49, 57, 57, 57, 57, 57, 57, 57, 69, 69, 75, 75, 75, 75, 75, 75, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 105, 105, 105 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
David L. Applegate, Hans Havermann, Bob Selcoe, Vladimir Shevelev, N. J. A. Sloane, and Reinhard Zumkeller, The Yellowstone Permutation, arXiv preprint arXiv:1501.01669 [math.NT], 2015 and J. Int. Seq. 18 (2015) 15.6.7.
MATHEMATICA
terms = 70; max = 2 terms;
f[lst_] := Block[{k = 4}, While[GCD[lst[[-2]], k] == 1 || GCD[lst[[-1]], k] > 1 || MemberQ[lst, k], k++]; Append[lst, k]];
A098550 = Nest[f, {1, 2, 3}, max-3];
sel = Select[Transpose[{Range[max], A098550}], PrimeQ[#[[2]]]&][[All, 1]]+2;
A251542 = A098550[[sel]]/A098550[[sel-2]] ;
a[n_] := For[k = 1, k <= max, k = k+2, If[CompositeQ[k] && FreeQ[A098550[[1 ;; n]], k] && FreeQ[A251542, k], Return[k]]];
Table[a[n], {n, 1, terms}] (* Jean-François Alcover, Dec 06 2018, after Robert G. Wilson v in A098550 *)
PROG
(Haskell)
import Data.List (delete); import Data.List.Ordered (minus)
a251558 n = a251558_list !! (n-1)
a251558_list = 9 : 9 : 9 : f 2 3 [4..] (tail a014076_list) where
f u v ws zs = g ws where
g (x:xs) = if gcd x u > 1 && gcd x v == 1
then y : f v x (delete x ws) ys else g xs
where ys@(y:_) = zs `minus` [x]
-- Reinhard Zumkeller, Mar 11 2015
CROSSREFS
Cf. A014076.
Sequence in context: A290238 A286833 A216852 * A251559 A068395 A245429
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Dec 23 2014
STATUS
approved

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Last modified March 1 22:23 EST 2024. Contains 370443 sequences. (Running on oeis4.)