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A177958
a(n) = n for n <= 6; for n > 6, a(n) is the smallest number not already used such that gcd(a(n), a(n-1)) >= 6.
2
1, 2, 3, 4, 5, 6, 12, 18, 9, 27, 36, 24, 8, 16, 32, 40, 10, 20, 30, 15, 45, 54, 42, 7, 14, 21, 28, 35, 49, 56, 48, 60, 50, 25, 75, 90, 63, 70, 77, 11, 22, 33, 44, 55, 66, 72, 64, 80, 88, 96, 78, 13, 26, 39, 52, 65, 91, 84, 98, 105
OFFSET
1,2
COMMENTS
A permutation of the natural numbers.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..10000 (first 2000 terms from Ivan Neretin)
J. C. Lagarias, E. M. Rains and N. J. A. Sloane, The EKG sequence, Exper. Math. 11 (2002), 437-446.
J. C. Lagarias, E. M. Rains and N. J. A. Sloane, The EKG sequence, arXiv:math/0204011 [math.NT], 2002.
MAPLE
ina:= proc(n) evalb(n<7) end:
a:= proc(n) option remember;
local k;
if n<7 then n
else for k while ina(k) or igcd (k, a(n-1))<6 do od;
ina(k):= true; k
fi
end;
seq(a(n), n=1..60);
MATHEMATICA
t=Range[6]; Do[k=7; While[MemberQ[t, k] || GCD[t[[-1]], k] < 6, k++]; AppendTo[t, k], {n, 7, 100}]; t
PROG
(Python)
from sympy import gcd
l=list(range(1, 7))
for n in range(6, 101):
k=7
while k in l or gcd(l[n - 1], k)<6: k+=1
l.append(k)
print(l) # Indranil Ghosh, Jun 27 2017
CROSSREFS
Sequence in context: A367147 A322570 A108320 * A032941 A273733 A218345
KEYWORD
nonn,easy
AUTHOR
Jonathan Vos Post, Dec 16 2010
EXTENSIONS
Edited by Alois P. Heinz, Dec 16 2010
STATUS
approved