cp's OEIS Frontend

A194593 Semiprimes s such that phi(s)/2 is prime.

%I A194593 #29 Sep 08 2022 08:45:58
%S A194593 9,10,14,22,46,94,118,166,214,334,358,454,526,694,718,766,934,958,
%T A194593 1006,1126,1174,1438,1678,1726,1774,1966,2038,2374,2566,2614,2638,
%U A194593 2734,2878,2974,3046,3238,3646,3814,4054,4078,4126,4198,4414,4894,4918,5158,5638,5758,5806,5926,5998
%N A194593 Semiprimes s such that phi(s)/2 is prime.
%C A194593 For n > 2, A001221(a(n)) = A001221(A000010(a(n))) = 2, and A008683(a(n)) = A008683(A000010(a(n))) = 1. - _Torlach Rush_, Aug 23 2018
%C A194593 For n > 1, A000010(a(n)) = A077065(n-1). - _Torlach Rush_, Sep 11 2018
%H A194593 Robert Israel, <a href="/A194593/b194593.txt">Table of n, a(n) for n = 1..10000</a>
%F A194593 a(n) = 2*A005385(n-1), n>1.
%F A194593 a(n) = 4*A005384(n-1) + 2, n > 1. - _Michel Marcus_, Apr 02 2020
%p A194593 9, 10, op(select(s -> isprime(s/2) and isprime((s-2)/4), [seq(s,s=6..10000,8)])); # _Robert Israel_, Apr 06 2016
%t A194593 Select[Range@ 6000, PrimeOmega@ # == 2 && PrimeQ[EulerPhi[#]/2] &] (* _Michael De Vlieger_, Apr 06 2016 *)
%o A194593 (PARI) isok(n) = (bigomega(n)== 2) && isprime(eulerphi(n)/2); \\ _Michel Marcus_, Apr 06 2016
%o A194593 (Magma) [9] cat [2*p: p in PrimesUpTo(3000) | IsPrime((p - 1) div 2)]; // _Vincenzo Librandi_, Aug 25 2018
%Y A194593 Cf. A000010, A001358, A005384, A005385, A079148, A065966.
%K A194593 nonn
%O A194593 1,1
%A A194593 _Juri-Stepan Gerasimov_, Aug 30 2011
%E A194593 Corrected by _R. J. Mathar_, Oct 13 2011