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

Terms

a(0) =3a(1) =10a(2) =31a(3) =94a(4) =283a(5) =850a(6) =2551a(7) =7654a(8) =22963a(9) =68890a(10) =206671a(11) =620014a(12) =1860043a(13) =5580130a(14) =16740391a(15) =50221174a(16) =150663523a(17) =451990570a(18) =1355971711a(19) =4067915134a(20) =12203745403a(21) =36611236210a(22) =109833708631a(23) =329501125894a(24) =988503377683