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

Terms

a(0) =0a(1) =1a(2) =8a(3) =41a(4) =176a(5) =689a(6) =2552a(7) =9113a(8) =31712a(9) =108257a(10) =364136a(11) =1210505a(12) =3985808a(13) =13020305a(14) =42249560a(15) =136314617a(16) =437641664a(17) =1399018433a(18) =4455335624a(19) =14140847849a(20) =44747066480a(21) =141214768241a(22) =444565011128