a(n) is the length of stage n in A137843.

Terms

a(0) =1a(1) =3a(2) =6a(3) =13a(4) =27a(5) =54a(6) =109a(7) =218a(8) =437a(9) =874a(10) =1749a(11) =3499a(12) =6998a(13) =13997a(14) =27995a(15) =55991a(16) =111982a(17) =223965a(18) =447931a(19) =895862a(20) =1791725a(21) =3583450a(22) =7166901a(23) =14333802a(24) =28667605a(25) =57335211a(26) =114670422a(27) =229340845a(28) =458681691a(29) =917363382