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A354356 Numbers k such that sigma(k) is 3-smooth (has no larger prime factors than 3).

%I A354356 #15 May 25 2022 05:45:02
%S A354356 1,2,3,5,6,7,10,11,14,15,17,21,22,23,30,31,33,34,35,42,46,47,51,53,55,
%T A354356 62,66,69,70,71,77,85,93,94,102,105,106,107,110,115,119,127,138,141,
%U A354356 142,154,155,159,161,165,170,186,187,191,210,213,214,217,230,231,235,238,253,254,255,265,282,310,318,321,322,329
%N A354356 Numbers k such that sigma(k) is 3-smooth (has no larger prime factors than 3).
%C A354356 The prime terms in this sequence are in A005105. - _Amiram Eldar_, May 25 2022
%H A354356 Amiram Eldar, <a href="/A354356/b354356.txt">Table of n, a(n) for n = 1..10000</a>
%H A354356 <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>
%t A354356 Select[Range[330], Max@ FactorInteger[DivisorSigma[1, #]][[All, 1]] <= 3 &] (* _Michael De Vlieger_, May 24 2022 *)
%o A354356 (PARI)
%o A354356 A065333(n) = ((3^valuation(n, 3)<<valuation(n, 2))==n); \\ From A065333
%o A354356 A354355(n) = A065333(sigma(n));
%o A354356 isA354356(n) = A354355(n);
%Y A354356 Cf. A000203, A065333, A354355 (characteristic function).
%Y A354356 Cf. A005105, A046528, A354357 (subsequences).
%Y A354356 Cf. also A122254.
%K A354356 nonn
%O A354356 1,2
%A A354356 _Antti Karttunen_, May 24 2022