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

Terms

a(0) =0a(1) =1a(2) =3a(3) =12a(4) =40a(5) =150a(6) =546a(7) =2128a(8) =8352a(9) =34380a(10) =144100a(11) =626736a(12) =2784288a(13) =12753832a(14) =59692920a(15) =286857600a(16) =1407536896a(17) =7069630608a(18) =36217682352a(19) =189489626560