Number of simple involutions of length n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =0a(4) =0a(5) =2a(6) =4a(7) =10a(8) =36a(9) =126a(10) =454a(11) =1744a(12) =6892a(13) =28486a(14) =120962a(15) =531660a(16) =2398248a(17) =11127674a(18) =52888998a(19) =257570104a(20) =1282439216