Series-parallel numbers.

Terms

a(0) =8a(1) =52a(2) =288a(3) =1424a(4) =6648a(5) =29700a(6) =128800a(7) =545600a(8) =2269672a(9) =9303140a(10) =37672216a(11) =150998016a(12) =599988696a(13) =2366216164a(14) =9270987656a(15) =36116062832a(16) =139978757920a(17) =540069059028