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URL: https://oeis.org/A393439

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A393439
Index of 2*n in A390314, or -1 if 2*n does not appear.
2
2, 3, 4, 6, 5, 7, 9, 8, 13, 17, 12, 10, 14, 18, 11, 21, 15, 16, 19, 20, 22, 25, 23, 24, 32, 26, 29, 27, 30, 28, 34, 33, 31, 45, 35, 38, 43, 48, 39, 40, 46, 36, 62, 44, 37, 49, 42, 41, 52, 53, 50, 47, 51, 58, 54, 55, 65, 66, 77, 56, 59, 67, 60, 76, 69, 57, 78
OFFSET
1,1
PROG
(Python)
from itertools import count
from math import gcd
from sympy import primefactors
def A393439(n):
c, aset, a, b, s, p = set(), {1}, 1, 2, 1, n<<1
for a in count(2):
for k in count(b, 2):
if k not in aset:
m = s+k
if gcd(m, k) == 1 and all(m%p for p in c):
if k == p:
return a
a = k
aset.add(a)
s += a
c |= set(primefactors(a))
while b in aset:
b += 2
break # Chai Wah Wu, Mar 10 2026
CROSSREFS
Sequence in context: A342798 A132284 A131966 * A360371 A194970 A194982
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Mar 10 2026.
EXTENSIONS
More terms from Chai Wah Wu, Mar 10 2026
STATUS
approved