a(n) is the length of stage n in A137844.

Terms

a(0) =1a(1) =3a(2) =7a(3) =15a(4) =30a(5) =61a(6) =123a(7) =247a(8) =495a(9) =991a(10) =1983a(11) =3967a(12) =7934a(13) =15869a(14) =31739a(15) =63479a(16) =126959a(17) =253919a(18) =507839a(19) =1015678a(20) =2031357a(21) =4062715a(22) =8125431a(23) =16250863a(24) =32501727a(25) =65003455a(26) =130006911a(27) =260013822a(28) =520027645a(29) =1040055291