%I #38 Jun 22 2018 10:40:43
%S 2,31,181,733,1777,8363,8369,19273,175333,175349,33952819,4377722977,
%T 4377723013,1242030992717,1242030992723
%N The largest prime in the first set of n consecutive primes for which p+4 is semiprime.
%C a(n) = last prime in the first run of n primes such that p+4 is semiprime for each prime p in the run. - _Sean A. Irvine_, Feb 13 2012
%C a(n) > 5 * 10^9 for n > 13.
%e a(2)=31 is the largest in a set of 2 consecutive primes {29,31}, and 29 + 4 = 33 = 3*11 and 31 + 4 = 35 = 5*7 are both semiprime. No smaller number has this property.
%e 59 is not in the sequence because although 47 + 4 = 51 = 3*17 and 53 + 4 = 57 = 3*19 are both semiprime, 59 + 4 = 63 = 3*3*7 is not.
%t With[{prs=Table[If[PrimeOmega[n+4]==2,1,0],{n,Prime[Range[21*10^5]]}]}, Prime[ #]&/@Flatten[Table[SequencePosition[prs,PadRight[{},n,1],1],{n,11}],1]][[All,2]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, May 10 2018 *)
%o (PARI) a(n) = {my(t = 0); forprime(p = 2, oo, if(bigomega(p + 4) == 2, t++; if(t==n, return(p)), t = 0))} \\ _David A. Corneth_, May 10 2018
%Y Cf. A001358, A137625, A137627, A137628.
%Y Subsequence of A289250.
%K more,nonn
%O 1,1
%A _Enoch Haga_, Jan 30 2008
%E a(11) from _Sean A. Irvine_, Feb 12 2012
%E a(1) corrected by _Harvey P. Dale_, May 10 2018
%E a(12)-a(13) from _David A. Corneth_, May 10 2018
%E a(14)-a(15) from _Giovanni Resta_, Jun 22 2018