A393439 Index of 2*n in A390314, or -1 if 2*n does not appear.

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

Links

Chai Wah Wu, Table of n, a(n) for n = 1..10000

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

Cf. A390314, A393440.

Sequence in context: A342798 A132284 A131966 * A360371 A194970 A194982

Adjacent sequences: A393436 A393437 A393438 * A393440 A393441 A393442

Keyword

nonn

Author

N. J. A. Sloane, Mar 10 2026.

Extensions

More terms from Chai Wah Wu, Mar 10 2026

Status

approved