A005099 - OEIS

A005099

(( Primes == -1 (mod 4) ) + 1)/4.

1, 2, 3, 5, 6, 8, 11, 12, 15, 17, 18, 20, 21, 26, 27, 32, 33, 35, 38, 41, 42, 45, 48, 50, 53, 56, 57, 60, 63, 66, 68, 71, 77, 78, 83, 87, 90, 92, 95, 96, 105, 108, 110, 111, 116, 117, 120, 122, 123, 125, 126, 131, 137, 141, 143, 147, 150, 152, 155, 158, 161, 162, 165

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

Numbers k such that 4*k - 1 is prime. - Michel Marcus, Dec 10 2015

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

FORMULA

a(n) = (A002145(n) + 1)/4. - R. J. Mathar, Jun 07 2011

MATHEMATICA

lst={}; Do[p=4*n-1; If[PrimeQ[p], AppendTo[lst, n]], {n, 0, 10^3}]; lst (* Vladimir Joseph Stephan Orlovsky, Sep 10 2008 *)

(Select[Prime@ Range@ 120, Mod[#, 4] == 3 &] + 1)/4 (* or *) Select[Range@165, PrimeQ[4 # - 1] &] (* Michael De Vlieger, Dec 10 2015 *)