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
KEYWORD
nonn
AUTHOR
Leroy Quet, Aug 20 2006
EXTENSIONS
Extended by Ray Chandler, Aug 22 2006
STATUS
approved