a(0)=1=a(1), a(2)=2, a(3)=5; thereafter a(n+3)=4*a(n+2)-4*a(n+1)+2*a(n) for n>=1.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =14a(5) =40a(6) =114a(7) =324a(8) =920a(9) =2612a(10) =7416a(11) =21056a(12) =59784a(13) =169744a(14) =481952a(15) =1368400a(16) =3885280a(17) =11031424a(18) =31321376a(19) =88930368a(20) =252498816a(21) =716916544a(22) =2035531648a(23) =5779458048a(24) =16409538688a(25) =46591385856a(26) =132286304768a(27) =375598753024