A251558 a(n) = smallest odd number not in {A098550(1), A098550(2), ..., A098550(n)} which is neither a prime nor a term of A251542.

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

Offset

1,1

Links

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

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. A098550, A251542, A251546, A251557, A251559.

Cf. A014076.

Sequence in context: A290238 A286833 A216852 * A251559 A068395 A245429

Adjacent sequences: A251555 A251556 A251557 * A251559 A251560 A251561

Keyword

nonn

Author

N. J. A. Sloane, Dec 23 2014

Status

approved