A387721 Numbers k for which A107758(k) > 2*k, where A107758 is sigma+, multiplicative function with a(p^e) = 1+sigma(p^e).

6, 10, 12, 14, 15, 18, 20, 21, 22, 24, 26, 28, 30, 34, 36, 38, 40, 42, 44, 45, 46, 48, 50, 52, 54, 56, 58, 60, 62, 66, 68, 70, 72, 74, 75, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100, 102, 104, 105, 106, 108, 110, 112, 114, 116, 118, 120, 122, 124, 126, 130, 132, 134, 135, 136, 138, 140, 142, 144, 146, 148

Offset

1,1

Comments

Note that in contrast to analogous sequences like A005101, A246282, A387711, here it is not guaranteed that multiples of any term are also terms. For example, 21 is a term, but 3*21 = 63 is in A052396. Also 15, 45, 75 are in this sequence, but their multiple 225 is in A387720.

Links

Antti Karttunen, Table of n, a(n) for n = 1..10000

Prog

(PARI)
A107758(n) =  { my(f = factor(n)); prod(k=1, #f~, 1+sigma(f[k, 1]^f[k, 2])); };
is_A387721(n) = (A107758(n)>(2*n));

Crossrefs

Cf. A107758.

Complement of (A052396 U A387720).

Disjoint union of A387722 and A387723.

Positions of positive terms in A387725.

Cf. also A005101 (subsequence), A246282, A387711.

Sequence in context: A389065 A386207 A324455 * A327476 A007774 A030231

Adjacent sequences: A387718 A387719 A387720 * A387722 A387723 A387724

Keyword

nonn

Author

Antti Karttunen, Sep 06 2025

Status

approved