A324909 Odd numbers k for which sigma(k^2) == 3 (mod 4).

5, 13, 15, 17, 29, 35, 37, 39, 41, 45, 51, 53, 55, 61, 73, 87, 89, 91, 95, 97, 101, 105, 109, 111, 113, 115, 117, 119, 123, 125, 135, 137, 143, 149, 153, 155, 157, 159, 165, 173, 181, 183, 187, 193, 197, 203, 215, 219, 229, 233, 235, 241, 245, 247, 257, 259, 261, 267, 269, 273, 277, 281, 285, 287, 291, 293, 295, 299, 303, 313

Offset: 1

Author

Antti Karttunen, Apr 20 2019

Keywords

• nonn

Comments

Odd numbers with an odd number of prime factors congruent to 1 modulo 4, or odd members of A209922. - Charlie Neder, Apr 24 2019

Links

• Antti Karttunen, Table of n, a(n) for n = 1..10000
• Index entries for sequences related to sigma(n).

Crossrefs

Cf. A000203, A000196, A324899.

Cf. A325312 (a subsequence).

Cf. A209922 (a supersequence).

Programs

Mathematica

Select[Range[1, 315, 2], Mod[DivisorSigma[1, #^2], 4] == 3 &] (* Amiram Eldar, Apr 05 2024 *)

PARI

isA324909(n) = ((n%2)&&3==(sigma(n*n)%4));

Formula

a(n) = sqrt(A324899(n)).