A388015 - OEIS

A388015

Numbers k that are odd multiples of 3 and non-multiples of 5, and for which sigma(k^2)/(k^2) >= 5/3.

21, 63, 147, 189, 231, 273, 357, 399, 429, 441, 483, 561, 567, 609, 627, 651, 693, 777, 819, 861, 903, 987, 1029, 1071, 1113, 1197, 1239, 1281, 1287, 1323, 1407, 1449, 1491, 1533, 1617, 1659, 1683, 1701, 1743, 1827, 1869, 1881, 1911, 1953, 1989, 2037, 2079, 2121, 2163, 2223, 2247, 2277, 2289, 2331, 2373, 2457, 2499

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OFFSET

1,1

LINKS

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

Index entries for sequences related to sigma(n).

FORMULA

a(n) = A000196(A388016(n)).

MATHEMATICA

Select[Range[1, 5000, 2], Divisible[#, 3] && !Divisible[#, 5] && 3*DivisorSigma[1, #^2] >= 5*#^2 &] (* Paolo Xausa, Sep 14 2025 *)

PROG

(PARI) is_A388015(n) = ((n%2) && !(n%3) && (n%5) && ((sigma(n^2)/(n^2))>=(5/3)));