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A098548 a(n) = n if n <= 3, otherwise the smallest number > a(n-1) having at least one common factor with a(n-2) but none with a(n-1). 21
1, 2, 3, 4, 9, 10, 21, 22, 27, 28, 33, 34, 39, 40, 51, 52, 57, 58, 63, 64, 69, 70, 81, 82, 87, 88, 93, 94, 99, 100, 111, 112, 117, 118, 123, 124, 129, 130, 141, 142, 147, 148, 153, 154, 159, 160, 171, 172, 177, 178, 183, 184, 189, 190, 201, 202, 207, 208, 213, 214 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The number a(n) is even if and only if n is even. If n>=1, then a(2n) = a(2n-1) + 1. If n>=2, then a(2n+1) - a(2n) >= 5. As a consequence, if n>=15, then a(n) > 3n. - Benoit Jubin, Dec 07 2014

A098549(n) = a(a(n)).

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.

Reinhard Zumkeller, Table of n, a(n) for n = 1..100000

MAPLE

x2 := 0: for n from 1 to 1000 do x := x2 + 1: while (n >= 4 and (gcd(x, x2) > 1 or gcd(x, x1) = 1)) do x := x + 1: end do; print (n, x); x1 := x2: x2 := x: end do: # David Applegate, Nov 26 2014

MATHEMATICA

a := {1, 2, 3}; For[n = 4, n <= 1000, n++, If[GCD[n, a[[-1]]] == 1 && GCD[n, a[[-2]]] > 1, AppendTo[a, n]]]; a (* L. Edson Jeffery, Dec 04 2014 *)

PROG

(Haskell)

a098548 n = a098548_list !! (n-1)

a098548_list = 1 : 2 : 3 : f 2 3 [4..] where

   f u v (w:ws) = if gcd u w > 1 && gcd v w == 1

                     then w : f v w ws else f u v ws

-- Reinhard Zumkeller, Nov 21 2014

CROSSREFS

Cf. A098549, A098550.

Cf. A158478 (smallest prime factor), A251104 (largest prime factor), A251139 (number of distinct prime factors), A251141 (total number of prime factors), A251046 (squarefree part), A251090 (squarefree kernel).

Cf. also A251535 and A251536 (bisections), A251537, A251538, A251539 (jumps), A251540.

Sequence in context: A174437 A127150 A256189 * A076963 A081871 A329573

Adjacent sequences:  A098545 A098546 A098547 * A098549 A098550 A098551

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, Sep 14 2004

STATUS

approved

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Last modified September 29 15:17 EDT 2020. Contains 337432 sequences. (Running on oeis4.)