A389205 Positions of odd terms in A325973, where A325973 is the arithmetic mean of {sum of unitary divisors} and {sum of squarefree divisors}.

1, 2, 9, 18, 36, 49, 72, 81, 98, 121, 144, 162, 196, 242, 288, 324, 361, 392, 484, 529, 576, 648, 722, 729, 784, 961, 968, 1058, 1152, 1296, 1444, 1458, 1568, 1849, 1922, 1936, 2116, 2209, 2304, 2401, 2592, 2888, 2916, 3136, 3481, 3698, 3844, 3872, 4232, 4418, 4489, 4608, 4802, 5041, 5184, 5776, 5832, 6241, 6272

Offset

1,2

Comments

After 1, 2, all terms are of the form 2^h * p^2e, with h >= 0, and p an odd prime of the form 4u+3 (A002145).

Links

Antti Karttunen, Table of n, a(n) for n = 1..5005

Prog

(PARI)
A325973(n) = (1/2)*sumdiv(n, d, d*(issquarefree(d) + (1==gcd(d, n/d))));
is_A389205(k) = (A325973(k)%2);

Crossrefs

Setwise difference A028982 \ A389210.

Union of {1, 2} and the setwise difference A389197 \ A389199.

Cf. A002145, A325973, A389197, A389204.

Sequence in context: A294543 A295956 A296843 * A200085 A083708 A280588

Adjacent sequences: A389202 A389203 A389204 * A389206 A389207 A389208

Keyword

nonn

Author

Antti Karttunen, Oct 06 2025

Status

approved