a(n) = (7*3^n + 1)/2.

Terms

a(0) =4a(1) =11a(2) =32a(3) =95a(4) =284a(5) =851a(6) =2552a(7) =7655a(8) =22964a(9) =68891a(10) =206672a(11) =620015a(12) =1860044a(13) =5580131a(14) =16740392a(15) =50221175a(16) =150663524a(17) =451990571a(18) =1355971712a(19) =4067915135a(20) =12203745404a(21) =36611236211a(22) =109833708632a(23) =329501125895a(24) =988503377684