A087398 - OEIS

%I #13 Oct 01 2013 17:57:43

%S 13,103,1153,15013,255253,4849843,111546433,100280245063,

%T 152125131763603,16294579238595022363,278970415063349480483707693,

%U 11992411764462614086353260819346129198103,481473710367991963528473107950567214598209565303106537707981745633

%N Primes of the form primorial(P(k))/2-2.

%C Twinmorial numbers are the partial products of twin primes. Sum of reciprocals = 0.08756985926348207565388288916..

%C The next term (a(14)) has 174 digits. - _Harvey P. Dale_, Mar 30 2013

%H Charles R Greathouse IV, <a href="/A087398/b087398.txt">Table of n, a(n) for n = 1..19</a>

%F Twins 3*5 = 15 = p+2. p=13.

%t Select[#/2-2&/@Rest[FoldList[Times,1,Prime[Range[100]]]],PrimeQ] (* _Harvey P. Dale_, Mar 30 2013 *)

%o (PARI) twimorial(n) = { s=0; p=3; forprime(x=5,n, if(isprime(x-2),c1++); p=p*x; if(isprime(p-2), print1(p-2","); c2++; s+=1.0/(p-2); ) ); print(); print(s) }

%o (PARI) v=[];pr=1; forprime(p=3,2357,pr*=p; if(ispseudoprime(pr-2),v=concat(v,pr-2))) \\ _Charles R Greathouse IV_, Feb 14 2011

%Y Cf. A096177 primes k such that primorial(k)/2+2 is prime, A096178 primes of the form primorial(k)/2+2, A096547 Primes k such that primorial(k)/2-2 is prime, A067024 smallest p+2 that has n distinct prime factors, A014545 primorial primes.

%K nonn

%O 1,1

%A _Cino Hilliard_, Oct 21 2003

%E Description corrected by _Hugo Pfoertner_, Jun 25 2004

%E One more term (a(13)) added by _Harvey P. Dale_, Mar 30 2013