a(0)=0, a(1)=1, a(2)=4, a(3)=13; thereafter a(n+3)=4*a(n+2)-4*a(n+1)+2*a(n) for n>=1.

Terms

a(0) =0a(1) =1a(2) =4a(3) =13a(4) =38a(5) =108a(6) =306a(7) =868a(8) =2464a(9) =6996a(10) =19864a(11) =56400a(12) =160136a(13) =454672a(14) =1290944a(15) =3665360a(16) =10407008a(17) =29548480a(18) =83896608a(19) =238206528a(20) =676336640a(21) =1920313664a(22) =5452321152a(23) =15480703232a(24) =43954155648a(25) =124798451968a(26) =354338591744