Series-parallel numbers.

Terms

a(0) =2a(1) =8a(2) =34a(3) =136a(4) =538a(5) =2080a(6) =7970a(7) =30224a(8) =113874a(9) =426520a(10) =1590338a(11) =5906640a(12) =21866794a(13) =80725280a(14) =297292610a(15) =1092539736a(16) =4007553922a(17) =14675748416a(18) =53663197698a(19) =195962433328a(20) =714734393378