A393440 Index of 2*prime(n) in A390314, or -1 if 2*prime(n) does not appear.

3, 4, 5, 9, 12, 14, 15, 19, 23, 30, 34, 43, 46, 62, 42, 51, 77, 59, 78, 63, 70, 84, 87, 92, 104, 98, 108, 107, 140, 103, 134, 125, 137, 126, 153, 151, 165, 182, 164, 180, 184, 233, 190, 205, 211, 198, 218, 222, 226, 228, 231, 253, 249, 259, 250, 263, 275, 278, 294, 282, 332, 284

Offset

1,1

Links

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

Prog

(Python)
from math import gcd
from itertools import count
from sympy import prime, primefactors
def A393440(n):
    c, aset, a, b, s, p = set(), {1}, 1, 2, 1, prime(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, A393439.

Sequence in context: A243654 A063953 A267755 * A028952 A361997 A108018

Adjacent sequences: A393437 A393438 A393439 * A393441 A393442 A393443

Keyword

nonn

Author

N. J. A. Sloane, Mar 10 2026

Extensions

More terms from Michael De Vlieger, Mar 10 2026

More terms from Chai Wah Wu, Mar 10 2026

Status

approved