a(n) = 11*3^n-1.

Terms

a(0) =10a(1) =32a(2) =98a(3) =296a(4) =890a(5) =2672a(6) =8018a(7) =24056a(8) =72170a(9) =216512a(10) =649538a(11) =1948616a(12) =5845850a(13) =17537552a(14) =52612658a(15) =157837976a(16) =473513930a(17) =1420541792a(18) =4261625378a(19) =12784876136a(20) =38354628410a(21) =115063885232a(22) =345191655698