A351535 - OEIS

A351535

Odd numbers k that are not multiples of 3 and for which sigma(k) is congruent to 2 modulo 4.

5, 13, 17, 29, 37, 41, 53, 61, 73, 89, 97, 101, 109, 113, 137, 149, 157, 173, 181, 193, 197, 229, 233, 241, 245, 257, 269, 277, 281, 293, 313, 317, 325, 337, 349, 353, 373, 389, 397, 401, 409, 421, 425, 433, 449, 457, 461, 509, 521, 541, 557, 569, 577, 593, 601, 605, 613, 617, 637, 641, 653, 661, 673, 677, 701, 709

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OFFSET

1,1

COMMENTS

a(25) = 245 = 5* 7^2 is the first term that is not prime.

LINKS

Table of n, a(n) for n=1..66.

Index entries for sequences related to sigma(n)

MATHEMATICA

Select[Range[1, 700, 2], !Divisible[#, 3] && Mod[DivisorSigma[1, #], 4] == 2 &] (* Amiram Eldar, Feb 13 2022 *)

PROG

(PARI) isA351535(n) = ((n%2) && (0!=(n%3)) && (2 == (sigma(n)%4)));