Number of descents in all involutions of {1,2,...,n}.

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =15a(5) =52a(6) =190a(7) =696a(8) =2674a(9) =10480a(10) =42732a(11) =178480a(12) =770836a(13) =3411024a(14) =15538120a(15) =72446752a(16) =346550520a(17) =1694394496a(18) =8477167504a(19) =43287312960a(20) =225707308912