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

Terms

a(0) =1a(1) =8a(2) =36a(3) =148a(4) =596a(5) =2388a(6) =9556a(7) =38228a(8) =152916a(9) =611668a(10) =2446676a(11) =9786708a(12) =39146836a(13) =156587348a(14) =626349396a(15) =2505397588a(16) =10021590356a(17) =40086361428a(18) =160345445716a(19) =641381782868