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

Terms

a(0) =0a(1) =8a(2) =32a(3) =104a(4) =320a(5) =968a(6) =2912a(7) =8744a(8) =26240a(9) =78728a(10) =236192a(11) =708584a(12) =2125760a(13) =6377288a(14) =19131872a(15) =57395624a(16) =172186880a(17) =516560648a(18) =1549681952a(19) =4649045864a(20) =13947137600a(21) =41841412808a(22) =125524238432a(23) =376572715304