Number of n-level ladder expressions with A001622.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =7a(5) =15a(6) =35a(7) =81a(8) =195a(9) =473a(10) =1170a(11) =2920a(12) =7378a(13) =18787a(14) =48242a(15) =124658a(16) =324095a(17) =846872a(18) =2223352a(19) =5861011a(20) =15508423a(21) =41173560a(22) =109648734