Numbers k for which 2*k-1, 4*k-1 and 8*k-1 are primes.

Terms

a(0) =3a(1) =6a(2) =21a(3) =45a(4) =90a(5) =180a(6) =255a(7) =360a(8) =510a(9) =516a(10) =615a(11) =705a(12) =726a(13) =741a(14) =756a(15) =906a(16) =945a(17) =951a(18) =966a(19) =1230a(20) =1350a(21) =1410a(22) =1725a(23) =1746a(24) =1770a(25) =1911a(26) =1956a(27) =2541a(28) =2700a(29) =2721