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

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, 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-Fran├žois 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

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Last modified December 16 04:05 EST 2019. Contains 330013 sequences. (Running on oeis4.)