a(n) is the least k such that k*(k+1)*Mersenne-prime(n)+1 is prime.

Terms

a(0) =1a(1) =2a(2) =3a(3) =12a(4) =8a(5) =3a(6) =5a(7) =14a(8) =17a(9) =69a(10) =189a(11) =42a(12) =392a(13) =167a(14) =377a(15) =12a(16) =2007a(17) =434a(18) =744a(19) =705a(20) =1089a(21) =1109a(22) =7833a(23) =7328a(24) =1271a(25) =192a(26) =6770a(27) =2379