Number of separable involutions.

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =24a(5) =64a(6) =166a(7) =456a(8) =1234a(9) =3454a(10) =9600a(11) =27246a(12) =77132a(13) =221336a(14) =635078a(15) =1839000a(16) =5331274a(17) =15555586a(18) =45465412a(19) =133517130a(20) =392841336a(21) =1160033656a(22) =3432015726a(23) =10182891552a(24) =30267591290a(25) =90177226062a(26) =269117947728a(27) =804699330974