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

Terms

a(0) =12a(1) =34a(2) =100a(3) =298a(4) =892a(5) =2674a(6) =8020a(7) =24058a(8) =72172a(9) =216514a(10) =649540a(11) =1948618a(12) =5845852a(13) =17537554a(14) =52612660a(15) =157837978a(16) =473513932a(17) =1420541794a(18) =4261625380a(19) =12784876138a(20) =38354628412a(21) =115063885234a(22) =345191655700