a(n) = (6^n + 2^n)/2.

Terms

a(0) =1a(1) =4a(2) =20a(3) =112a(4) =656a(5) =3904a(6) =23360a(7) =140032a(8) =839936a(9) =5039104a(10) =30233600a(11) =181399552a(12) =1088393216a(13) =6530351104a(14) =39182090240a(15) =235092508672