This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A369564 #7 Jan 26 2024 08:34:41
%S A369564 1,9,27,49,81,121,243,343,361,441,529,729,961,1089,1323,1331,1849,
%T A369564 2187,2209,2401,3087,3249,3267,3481,3969,4489,4761,5041,5929,6241,
%U A369564 6561,6859,6889,8649,9261,9747,9801,10609,11449,11907,11979,12167,14283,14641,16129,16641
%N A369564 Powerful numbers whose prime factors are all of the form 4*k + 3.
%C A369564 Closed under multiplication.
%H A369564 Amiram Eldar, <a href="/A369564/b369564.txt">Table of n, a(n) for n = 1..10000</a>
%H A369564 <a href="/index/Pow#powerful">Index entries for sequences related to powerful numbers</a>.
%F A369564 Sum_{n>=1} 1/a(n) = Product_{primes p == 3 (mod 4)} (1 + 1/(p*(p-1))) = 3*A013661*A334426/(4*A175647) = 1.2161513254... .
%t A369564 q[n_] := n == 1 || AllTrue[FactorInteger[n], Mod[First[#], 4] == 3 && Last[#] > 1 &]; Select[Range[20000], q]
%o A369564 (PARI) is(n) = {my(f = factor(n)); for(i = 1, #f~, if(f[i, 1]%4 != 3 || f[i, 2] == 1, return(0))); 1;}
%Y A369564 Intersection of A001694 and A004614.
%Y A369564 Similar sequence: A352492, A369563, A369565, A369566.
%Y A369564 Cf. A002145, A013661, A175647, A334426.
%K A369564 nonn
%O A369564 1,2
%A A369564 _Amiram Eldar_, Jan 26 2024