Bisection of A000031.

Terms

a(0) =1a(1) =3a(2) =6a(3) =14a(4) =36a(5) =108a(6) =352a(7) =1182a(8) =4116a(9) =14602a(10) =52488a(11) =190746a(12) =699252a(13) =2581428a(14) =9587580a(15) =35792568a(16) =134219796a(17) =505294128a(18) =1908881900a(19) =7233642930a(20) =27487816992a(21) =104715443852a(22) =399822505524