4^n + 3^n - 1.

Terms

a(0) =1a(1) =6a(2) =24a(3) =90a(4) =336a(5) =1266a(6) =4824a(7) =18570a(8) =72096a(9) =281826a(10) =1107624a(11) =4371450a(12) =17308656a(13) =68703186a(14) =273218424a(15) =1088090730a(16) =4338014016a(17) =17309009346a(18) =69106897224a(19) =276040168410