Sequence related to NOR bracketings.

Terms

a(0) =0a(1) =1a(2) =0a(3) =6a(4) =4a(5) =60a(6) =84a(7) =700a(8) =1440a(9) =8910a(10) =23100a(11) =120120a(12) =360360a(13) =1684956a(14) =5552064a(15) =24302520a(16) =85101456a(17) =357502860a(18) =1302562404a(19) =5333981796a(20) =19947127200a(21) =80408748420a(22) =305922388200