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 A121217 a(1)=1, a(2)=2, a(3)=3; for n > 3, a(n) is the smallest positive integer which does not occur earlier in the sequence and which is not coprime to a(n-2). 6
 1, 2, 3, 4, 6, 8, 9, 10, 12, 5, 14, 15, 7, 18, 21, 16, 24, 20, 22, 25, 11, 30, 33, 26, 27, 13, 36, 39, 28, 42, 32, 34, 38, 17, 19, 51, 57, 45, 48, 35, 40, 49, 44, 56, 46, 50, 23, 52, 69, 54, 60, 58, 55, 29, 65, 87, 70, 63, 62, 66, 31, 64, 93, 68, 72, 74, 75, 37, 78, 111, 76, 81 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Conjecture: this is a permutation of the positive integers, cf. A256618. - Reinhard Zumkeller, Apr 05 2015 The B-sequence mentioned in the Maple program is not in the OEIS. It is 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, ... - Alois P. Heinz, Feb 02 2019 LINKS N. J. A. Sloane, Table of n, a(n) for n = 1..20632 MAPLE # From N. J. A. Sloane, Apr 04 2015: A121217 gcd(A[n], A[n-2])>1 A=seq, for B see the COMMENTS N:= 60: # to get a(1) to a(n) where a(n+1) is the first term > N B:= Vector(N, datatype=integer[4]): for n from 1 to 3 do A[n]:= n: od: for n from 4 do for k from 4 to N do if B[k] = 0 and igcd(k, A[n-2]) > 1 then A[n]:= k; B[k]:= 1; break fi od: if k > N then break fi od: [seq(A[i], i=1..n-1)]; MATHEMATICA a = Range@ 3; Do[k = 4; While[Or[MemberQ[a, k], CoprimeQ[a[[i - 2]], k]], k++]; AppendTo[a, k], {i, 4, 72}]; a (* Michael De Vlieger, Aug 19 2017 *) PROG (Haskell) a121217 n = a121217_list !! (n-1) a121217_list = 1 : 2 : 3 : f 2 3 [4..] where f u v xs = g xs where g (w:ws) = if gcd w u > 1 then w : f v w (delete w xs) else g ws -- Reinhard Zumkeller, Apr 05 2015 CROSSREFS Cf. A064413, A121216, A251622, A256414 (indices of primes), A256419 (smoothed version). Cf. A256618 (conjectured inverse). Sequence in context: A184899 A039034 A047302 * A357992 A255582 A254077 Adjacent sequences: A121214 A121215 A121216 * A121218 A121219 A121220 KEYWORD nonn AUTHOR Leroy Quet, Aug 20 2006 EXTENSIONS Extended by Ray Chandler, Aug 22 2006 STATUS approved

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Last modified December 9 00:20 EST 2022. Contains 358698 sequences. (Running on oeis4.)