A251557 a(n) = 2 more than the largest even number among {A098550(1), A098550(2), ..., A098550(n)}.

2, 4, 4, 6, 6, 10, 10, 16, 16, 16, 16, 16, 16, 18, 18, 18, 18, 22, 22, 24, 24, 24, 24, 24, 28, 28, 30, 30, 34, 34, 34, 34, 34, 34, 36, 36, 38, 38, 38, 38, 40, 40, 40, 44, 44, 46, 46, 46, 46, 52, 52, 52, 52, 54, 54, 54, 54, 58, 58, 64, 64, 64, 64, 64, 66, 66, 70, 70, 70, 70, 70, 70, 70

Offset: 1

Author

N. J. A. Sloane, Dec 23 2014

Keywords

• nonn

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.

Crossrefs

Cf. A098550, A251546, A251558, A251559.

Programs

Haskell

a251557 n = a251557_list !! (n-1)
a251557_list = map (+ 2) $ tail $ scanl maxEven 0 a098550_list
               where maxEven u v = if even v then max u v else u
-- Reinhard Zumkeller, Mar 10 2015

Mathematica

terms = 100;
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}, terms - 3];
a[1] = 2; a[n_] := Max[Select[A098550[[1 ;; n]], EvenQ]] + 2;
Array[a, terms] (* Jean-François Alcover, Aug 02 2018, after Robert G. Wilson v *)