a(n)=a(n-1)+a(n-2)+a(n-3)+2a(n-4) with a(n)=n+1 for n<=3.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =11a(5) =22a(6) =43a(7) =84a(8) =171a(9) =342a(10) =683a(11) =1364a(12) =2731a(13) =5462a(14) =10923a(15) =21844a(16) =43691a(17) =87382a(18) =174763a(19) =349524a(20) =699051a(21) =1398102a(22) =2796203a(23) =5592404a(24) =11184811a(25) =22369622