A393440 - OEIS

%I #13 Mar 10 2026 19:26:57

%S 3,4,5,9,12,14,15,19,23,30,34,43,46,62,42,51,77,59,78,63,70,84,87,92,

%T 104,98,108,107,140,103,134,125,137,126,153,151,165,182,164,180,184,

%U 233,190,205,211,198,218,222,226,228,231,253,249,259,250,263,275,278,294,282,332,284

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

%H Chai Wah Wu, <a href="/A393440/b393440.txt">Table of n, a(n) for n = 1..10000</a>

%o (Python)

%o from math import gcd

%o from itertools import count

%o from sympy import prime, primefactors

%o def A393440(n):

%o     c, aset, a, b, s, p = set(), {1}, 1, 2, 1, prime(n)<<1

%o     for a in count(2):

%o         for k in count(b,2):

%o             if k not in aset:

%o                 m = s+k

%o                 if gcd(m,k) == 1 and all(m%p for p in c):

%o                     if k == p:

%o                         return a

%o                     a = k

%o                     aset.add(a)

%o                     s += a

%o                     c |= set(primefactors(a))

%o                     while b in aset:

%o                         b += 2

%o                     break # _Chai Wah Wu_, Mar 10 2026

%Y Cf. A390314, A393439.

%K nonn

%O 1,1

%A _N. J. A. Sloane_, Mar 10 2026

%E More terms from _Michael De Vlieger_, Mar 10 2026

%E More terms from _Chai Wah Wu_, Mar 10 2026