Pentanacci analog of A055502.

Terms

a(0) =0a(1) =2a(2) =3a(3) =7a(4) =13a(5) =29a(6) =59a(7) =113a(8) =223a(9) =439a(10) =877a(11) =1721a(12) =3389a(13) =6653a(14) =13093a(15) =25741a(16) =50599a(17) =99487a(18) =195581a(19) =384509a(20) =755959