a(n) = a(n-1) + 3*a(n-2) if n even, or 2*a(n-1) + 4*a(n-2) if n odd, starting with 0, 1.

Terms

a(0) =0a(1) =1a(2) =1a(3) =6a(4) =9a(5) =42a(6) =69a(7) =306a(8) =513a(9) =2250a(10) =3789a(11) =16578a(12) =27945a(13) =122202a(14) =206037a(15) =900882a(16) =1518993a(17) =6641514a(18) =11198493a(19) =48963042a(20) =82558521a(21) =360969210a(22) =608644773a(23) =2661166386a(24) =4487100705a(25) =19618866954a(26) =33080169069a(27) =144635805954a(28) =243876313161