A082131 Middle of semiprime triple: n-2, n, n+2 are semiprimes.

93, 121, 143, 185, 203, 215, 217, 219, 289, 301, 303, 321, 393, 413, 415, 471, 517, 535, 581, 669, 687, 697, 791, 815, 1057, 1079, 1135, 1137, 1139, 1147, 1167, 1205, 1255, 1315, 1345, 1347, 1349, 1385, 1387, 1389, 1391, 1403, 1563, 1641, 1687

Offset

1,1

Comments

All terms are odd. - David A. Corneth, Jul 16 2017

Links

Harvey P. Dale and David A. Corneth, Table of n, a(n) for n = 1..13739 (terms < 10^6, first 1000 terms from Harvey P. Dale)

Example

a(1) = 93 because 91 = 7*13, 93 = 3*31 and 95 = 5*19 are semiprime.

Mathematica

2#+1&/@Flatten[Position[Partition[If[PrimeOmega[#]==2, 1, 0]&/@Range[ 1, 1701, 2], 3, 1], _?(Total[#]==3&), {1}, Heads->False]] (* Harvey P. Dale, Jul 24 2013 *)

Prog

(PARI) isok(n) = (bigomega(n-2) == 2) && (bigomega(n)==2) && (bigomega(n+2) == 2); \\ Michel Marcus, Jul 16 2017
(PARI) list(lim)={my(u=primes(primepi(lim\3)), v=List(), t, res = List()); for(i=2, #u, for(j=i, #u, t=u[i]*u[j]; if(t>lim, break); listput(v, t))); listsort(v); for(i=1, #v-2, if(v[i]+4==v[i+2], listput(res, v[i+1]))); res} \\ adapted from _Charles R. Greathouse IV_ at A046315. - David A. Corneth, Jul 16 2017

Crossrefs

Cf. A001358, A046315, A082130.

Sequence in context: A039550 A167776 A099019 * A249300 A153684 A048257

Adjacent sequences: A082128 A082129 A082130 * A082132 A082133 A082134

Keyword

nonn

Author

Hugo Pfoertner, Apr 04 2003

Extensions

More terms from Jud McCranie, Apr 04 2003

Status

approved