c(p(n)) where p(k) is k-th prime including p(1)=1 and c(k) is k-th composite number.

Terms

a(0) =4a(1) =6a(2) =8a(3) =10a(4) =14a(5) =20a(6) =22a(7) =27a(8) =30a(9) =35a(10) =44a(11) =46a(12) =54a(13) =58a(14) =62a(15) =66a(16) =75a(17) =82a(18) =85a(19) =92a(20) =96a(21) =99a(22) =108a(23) =114a(24) =120a(25) =129a(26) =134a(27) =136a(28) =142a(29) =144