A124485 Numbers n such that 2n-1 and 4n-1 are primes.

2, 3, 6, 12, 15, 21, 27, 42, 45, 57, 66, 87, 90, 96, 117, 120, 126, 141, 147, 180, 210, 216, 222, 246, 255, 297, 321, 327, 330, 342, 360, 372, 381, 405, 456, 477, 507, 510, 516, 525, 552, 612, 615, 645, 705, 720, 726, 741, 750, 756, 780, 792, 801, 867, 906, 945

Offset: 1

Author

Artur Jasinski, Nov 04 2006

Keywords

• nonn

Links

• Muniru A Asiru, Table of n, a(n) for n = 1..5000

Crossrefs

Cf. A005384 (Sophie Germain primes).

Cf. A124486, A124487, A124488, A124489, A124490, A124491, A124492.

Programs

GAP

Filtered([1..1000],p->IsPrime(2*p-1) and IsPrime(4*p-1)); # Muniru A Asiru, Jul 19 2018

Maple

select(k->isprime(2*k-1) and isprime(4*k-1),[$1..1000]); # Muniru A Asiru, Jul 19 2018

Mathematica

Select[Range[1000], And @@ PrimeQ /@ ({2, 4}*# - 1) &] (* Ray Chandler, Nov 21 2006 *)

Formula

a(n) = (A005384(n+1) + 1)/2. - Hilko Koning, Jul 19 2018

Extensions

Extended by Ray Chandler, Nov 21 2006