A176256 - OEIS

%I #22 Apr 24 2021 08:44:35

%S 9,21,33,45,49,57,69,77,81,93,105,117,121,129,133,141,153,161,165,177,

%T 189,201,209,213,217,225,237,249,253,261,273,285,297,301,309,321,329,

%U 333,341,345,357,361,369,381,393,405,413,417,429,437,441,453,465,469

%N Numbers of the form 4k+1 with least prime divisor of the form 4m-1.

%C By definition, all terms are composite numbers.

%C Cannot be the hypotenuse of a primitive Pythagorean triangle. - _Robert G. Wilson v_, Mar 16 2014

%H Vincenzo Librandi, <a href="/A176256/b176256.txt">Table of n, a(n) for n = 1..1000</a>

%t fQ[n_] := Mod[ n, 4] == 1 && Mod[ FactorInteger[n][[1, 1]], 4] == 3; Select[Range@470, fQ] (* _Robert G. Wilson v_, Apr 08 2014 *)

%o (PARI) isok(n) = ((n % 4) == 1) && (f = factor(n)) && ((f[1, 1] % 4) == 3); \\ _Michel Marcus_, Mar 16 2014

%Y Cf. A176255, A002148, A002145, A016813, A004767.

%Y Complement of A020882 in 1 == Mod 4.

%K nonn

%O 1,1

%A _Vladimir Shevelev_, Apr 13 2010

%E More terms from _Michel Marcus_, Mar 16 2014