OFFSET
1,1
COMMENTS
Closed under multiplication.
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..10000
EXAMPLE
6 = 2*3 is included, as there is a prime factor of the form 4k+3 present.
75 = 3 * 5 * 5 is included, as there is a prime factor of the form 4k+3 present and the prime factor of the form 4k+1 (5) is present twice.
MATHEMATICA
Select[Range@ 150, AnyTrue[#, Mod[First@ #, 4] == 3 &] && NoneTrue[#, And[Mod[First@ #, 4] == 1, OddQ@ Last@ #] &] &@ FactorInteger@ # &] (* Michael De Vlieger, Feb 04 2016, Version 10 *)
PROG
(Scheme, with Antti Karttunen's IntSeq-library)
(define A268378 (MATCHING-POS 1 1 (lambda (n) (and (even? (A267113 n)) (not (zero? (A065339 n)))))))
(PARI) isok(n) = {my(f = factor(n), nb3 = 0); for (i=1, #f~, if (((f[i, 1] % 4) == 1) && (f[i, 2] % 2), return (0)); if ((f[i, 1] % 4) == 3, nb3++); ); return (nb3); } \\ Michel Marcus, Feb 04 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Antti Karttunen, Feb 03 2016
STATUS
approved