a(0) = 4; for n >= 1, a(n) = 2^n + 4.

Terms

a(0) =4a(1) =6a(2) =8a(3) =12a(4) =20a(5) =36a(6) =68a(7) =132a(8) =260a(9) =516a(10) =1028a(11) =2052a(12) =4100a(13) =8196a(14) =16388a(15) =32772a(16) =65540a(17) =131076a(18) =262148a(19) =524292a(20) =1048580a(21) =2097156a(22) =4194308a(23) =8388612a(24) =16777220a(25) =33554436a(26) =67108868a(27) =134217732a(28) =268435460a(29) =536870916