Series-parallel numbers.

Terms

a(0) =52a(1) =472a(2) =3224a(3) =18888a(4) =101340a(5) =511120a(6) =2465904a(7) =11496144a(8) =52165892a(9) =231557064a(10) =1009247192a(11) =4331502840a(12) =18346242492a(13) =76822836544a(14) =318485778848