A071821 Numbers whose largest prime factor is of the form 4k+1.

5, 10, 13, 15, 17, 20, 25, 26, 29, 30, 34, 37, 39, 40, 41, 45, 50, 51, 52, 53, 58, 60, 61, 65, 68, 73, 74, 75, 78, 80, 82, 85, 87, 89, 90, 91, 97, 100, 101, 102, 104, 106, 109, 111, 113, 116, 117, 119, 120, 122, 123, 125, 130, 135, 136, 137, 143, 145, 146, 148, 149

Offset: 1

Author

Benoit Cloitre, Jun 07 2002

Keywords

• easy
• nonn

Comments

Subsequence of A009003. - M. F. Hasler, Feb 06 2009

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A004431, A006530, A009000, A009003, A073503, A083025.

Programs

Maple

filter:= proc(n)
  max(numtheory:-factorset(n)) mod 4 = 1
end proc:
select(filter, [$1..200]); # Robert Israel, Sep 11 2020

Mathematica

Select[Range[2, 150], Mod[FactorInteger[#][[-1,1]], 4] == 1 &] (* Amiram Eldar, May 04 2022 *)

PARI

for(n=2, 200, if((component(component(factor(n), 1), omega(n))-1)%4==0, print1(n, ", ")))

PARI

for( n=2,99, vecmax(factor(n)[,1])%4==1 && print1(n",")) \\ M. F. Hasler, Feb 06 2009

Formula

Numbers k such that A006530(k) == 1 (mod 4).