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

Offset

1,1

Comments

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

The sequence shows a steady high percentage of primes. The percentages of the number of prime terms in the first 10^3, ..., 10^8 terms are 86.9, 86.6, 87.1, 87.8, 88.4, 88.8. Additionally, approx 99% of the composite terms indivisible by 5 belong to A348938. - Bill McEachen, Aug 21 2025

Links

Bill McEachen, Table of n, a(n) for n = 1..10000

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)));

Crossrefs

Intersection of A001651 and A191218. Complement of A351533 in A191218.

Cf. A002144 (subsequence).

Sequence in context: A231754 A387595 A175768 * A002144 A280084 A319287

Adjacent sequences: A351532 A351533 A351534 * A351536 A351537 A351538

Keyword

nonn

Author

Antti Karttunen, Feb 13 2022

Status

approved