a(n) is the smallest k such that prime(n)*k+1 is prime.

Terms

a(0) =1a(1) =2a(2) =2a(3) =4a(4) =2a(5) =4a(6) =6a(7) =10a(8) =2a(9) =2a(10) =10a(11) =4a(12) =2a(13) =4a(14) =6a(15) =2a(16) =12a(17) =6a(18) =4a(19) =8a(20) =4a(21) =4a(22) =2a(23) =2a(24) =4a(25) =6a(26) =6a(27) =6a(28) =10a(29) =2