a(n) = (7*2^n - 4(-1)^n)/3.

Terms

a(0) =1a(1) =6a(2) =8a(3) =20a(4) =36a(5) =76a(6) =148a(7) =300a(8) =596a(9) =1196a(10) =2388a(11) =4780a(12) =9556a(13) =19116a(14) =38228a(15) =76460a(16) =152916a(17) =305836a(18) =611668a(19) =1223340a(20) =2446676a(21) =4893356a(22) =9786708a(23) =19573420a(24) =39146836a(25) =78293676a(26) =156587348a(27) =313174700a(28) =626349396a(29) =1252698796