a(1) = 4; a(n) = smallest composite number of the form k*a(n-1) + 1.

Terms

a(0) =4a(1) =9a(2) =10a(3) =21a(4) =22a(5) =45a(6) =46a(7) =93a(8) =94a(9) =95a(10) =96a(11) =289a(12) =290a(13) =291a(14) =292a(15) =585a(16) =586a(17) =1173a(18) =1174a(19) =1175a(20) =1176a(21) =1177a(22) =1178a(23) =1179a(24) =1180a(25) =2361a(26) =2362a(27) =2363a(28) =2364a(29) =2365