A137626 The largest prime in the first set of n consecutive primes for which p+4 is semiprime.
2, 31, 181, 733, 1777, 8363, 8369, 19273, 175333, 175349, 33952819, 4377722977, 4377723013, 1242030992717, 1242030992723
Offset: 1
Examples
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.
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.
Programs
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Mathematica
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 *) -
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
Extensions
a(11) from Sean A. Irvine, Feb 12 2012
a(1) corrected by Harvey P. Dale, May 10 2018
a(12)-a(13) from David A. Corneth, May 10 2018
a(14)-a(15) from Giovanni Resta, Jun 22 2018
Comments