a(n) = 2*(10*3^n - 1).

Terms

a(0) =18a(1) =58a(2) =178a(3) =538a(4) =1618a(5) =4858a(6) =14578a(7) =43738a(8) =131218a(9) =393658a(10) =1180978a(11) =3542938a(12) =10628818a(13) =31886458a(14) =95659378a(15) =286978138a(16) =860934418a(17) =2582803258a(18) =7748409778a(19) =23245229338a(20) =69735688018a(21) =209207064058a(22) =627621192178