Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #21 May 04 2022 06:53:00
%S 5,10,13,15,17,20,25,26,29,30,34,37,39,40,41,45,50,51,52,53,58,60,61,
%T 65,68,73,74,75,78,80,82,85,87,89,90,91,97,100,101,102,104,106,109,
%U 111,113,116,117,119,120,122,123,125,130,135,136,137,143,145,146,148,149
%N Numbers whose largest prime factor is of the form 4k+1.
%C Subsequence of A009003. - _M. F. Hasler_, Feb 06 2009
%H Robert Israel, <a href="/A071821/b071821.txt">Table of n, a(n) for n = 1..10000</a>
%F Numbers k such that A006530(k) == 1 (mod 4).
%p filter:= proc(n)
%p max(numtheory:-factorset(n)) mod 4 = 1
%p end proc:
%p select(filter, [$1..200]); # _Robert Israel_, Sep 11 2020
%t Select[Range[2, 150], Mod[FactorInteger[#][[-1,1]], 4] == 1 &] (* _Amiram Eldar_, May 04 2022 *)
%o (PARI) for(n=2, 200, if((component(component(factor(n), 1), omega(n))-1)%4==0, print1(n, ", ")))
%o (PARI) for( n=2,99, vecmax(factor(n)[,1])%4==1 && print1(n",")) \\ _M. F. Hasler_, Feb 06 2009
%Y Cf. A004431, A006530, A009000, A009003, A073503, A083025.
%K easy,nonn
%O 1,1
%A _Benoit Cloitre_, Jun 07 2002