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

Terms

a(0) =1a(1) =6a(2) =26a(3) =100a(4) =363a(5) =1274a(6) =4372a(7) =14760a(8) =49205a(9) =162382a(10) =531438a(11) =1727180a(12) =5580127a(13) =17936130a(14) =57395624a(15) =182948560a(16) =581130729a(17) =1840247318a(18) =5811307330a(19) =18305618100a(20) =57531942611a(21) =180441092746a(22) =564859072956