a(n) = 4 * floor(28*2^n/15).

Terms

a(0) =4a(1) =12a(2) =28a(3) =56a(4) =116a(5) =236a(6) =476a(7) =952a(8) =1908a(9) =3820a(10) =7644a(11) =15288a(12) =30580a(13) =61164a(14) =122332a(15) =244664a(16) =489332a(17) =978668a(18) =1957340a(19) =3914680a(20) =7829364a(21) =15658732a(22) =31317468a(23) =62634936a(24) =125269876a(25) =250539756a(26) =501079516a(27) =1002159032a(28) =2004318068