cp's OEIS Frontend

A064418 a(1) = 1, a(2) = 2, a(3) = 3, a(4) = 4; for n > 4, a(n) = smallest number not already used such that gcd(a(n), a(n-1)) >= 4.

%I A064418 #18 Oct 18 2019 23:12:36
%S A064418 1,2,3,4,8,12,6,18,9,27,36,16,20,5,10,15,25,30,24,28,7,14,21,35,40,32,
%T A064418 44,11,22,33,55,45,50,60,42,48,52,13,26,39,65,70,49,56,63,54,66,72,64,
%U A064418 68,17,34,51,85,75,80,76,19,38,57,95,90,78,84,77,88,92,23,46,69,115,100,96
%N A064418 a(1) = 1, a(2) = 2, a(3) = 3, a(4) = 4; for n > 4, a(n) = smallest number not already used such that gcd(a(n), a(n-1)) >= 4.
%C A064418 A permutation of the natural numbers.
%H A064418 Reinhard Zumkeller, <a href="/A064418/b064418.txt">Table of n, a(n) for n = 1..10000</a>
%H A064418 J. C. Lagarias, E. M. Rains and N. J. A. Sloane, <a href="http://arXiv.org/abs/math.NT/0204011">The EKG sequence</a>, Exper. Math. 11 (2002), 437-446.
%H A064418 <a href="/index/Ed#EKG">Index entries for sequences related to EKG sequence</a>
%H A064418 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a>
%t A064418 A064418 = Range[4]; Do[ For[ an=5, True, an++, If[GCD[an, Last[A064418]] >= 4 && FreeQ[A064418, an], AppendTo[A064418, an]; Break[]]], {70}]; A064418 (* _Jean-François Alcover_, May 07 2012 *)
%o A064418 (Haskell)
%o A064418 import Data.List (delete)
%o A064418 a064418 n = a064418_list !! (n-1)
%o A064418 a064418_list = 1 : 2 : 3 : 4 : (f 4 [5..]) where
%o A064418    f :: Integer -> [Integer] -> [Integer]
%o A064418    f x xs = m : (f m $ delete m xs) where
%o A064418       m = head $ dropWhile ((< 4) . (gcd x)) xs
%o A064418 -- _Reinhard Zumkeller_, Feb 20 2011
%Y A064418 Cf. A064413. Inverse permutation: A064958.
%K A064418 nonn,nice,easy
%O A064418 1,2
%A A064418 Jonathan Ayres (Jonathan.ayres(AT)btinternet.com), Sep 30 2001
%E A064418 More terms from _Naohiro Nomoto_, Sep 30 2001