Number of certain noncrossing set partitions.

Terms

a(0) =1a(1) =4a(2) =12a(3) =37a(4) =118a(5) =387a(6) =1298a(7) =4433a(8) =15366a(9) =53924a(10) =191216a(11) =684114a(12) =2466428a(13) =8951945a(14) =32683230a(15) =119949945a(16) =442281030a(17) =1637618400a(18) =6086481720a(19) =22699003830a(20) =84918443220a(21) =318593346630