A391675 a(n) is the smallest k for which A243865(k) = n, or -1 if no such k exists.

1, -1, 3, 15, 40, 12, 24, 84, 60, 120, 180, 360, 420, 1560, 1440, 840, 1680, 3360, 4620, 10920, 2520, 5040, 7560, 15120, 18480, 36960, 57120, 42840, 27720, 115920, 85680, 55440, 83160, 65520, 131040, 332640, 196560, 277200, 415800, 360360, 526680, 1404480, 831600, 942480, 1053360, 1113840

Offset

0,3

Comments

a(1) = -1 (by definition).

Links

Table of n, a(n) for n=0..45.

Prog

(PARI) nbtd(n) = sumdiv(n, d, ((d>2) && !(n % (d-2))) || !(n % (d+2))); \\ A243865
a(n) = if (n==1, return(-1)); my(m=1); while (nbtd(m) != n, m++); m; \\ Michel Marcus, Feb 20 2026

Crossrefs

Cf. A243865, A243917.

Sequence in context: A146853 A183476 A297621 * A014309 A298049 A298884

Adjacent sequences: A391672 A391673 A391674 * A391676 A391677 A391678

Keyword

sign

Author

Juri-Stepan Gerasimov, Feb 19 2026

Extensions

More terms from Michel Marcus, Feb 20 2026

Status

approved