a(n)=a(n-1)+a(n-2)+a(n-3)+2a(n-4); a(0)=0,a(1)=1,a(2)=3,a(3)=7.

Terms

a(0) =0a(1) =1a(2) =3a(3) =7a(4) =11a(5) =23a(6) =47a(7) =95a(8) =187a(9) =375a(10) =751a(11) =1503a(12) =3003a(13) =6007a(14) =12015a(15) =24031a(16) =48059a(17) =96119a(18) =192239a(19) =384479a(20) =768955a(21) =1537911a(22) =3075823a(23) =6151647a(24) =12303291a(25) =24606583a(26) =49213167a(27) =98426335a(28) =196852667a(29) =393705335