a(n) = 2^n*tribonacci(n) or (2^n)*A001644(n+1).

Terms

a(0) =2a(1) =12a(2) =56a(3) =176a(4) =672a(5) =2496a(6) =9088a(7) =33536a(8) =123392a(9) =453632a(10) =1669120a(11) =6139904a(12) =22585344a(13) =83083264a(14) =305627136a(15) =1124270080a(16) =4135714816a(17) =15213527040a(18) =55964073984a(19) =205867974656a(20) =757300461568