Number of connected graphs with n edges with loops allowed.

Terms

a(0) =1a(1) =2a(2) =2a(3) =6a(4) =12a(5) =33a(6) =93a(7) =287a(8) =940a(9) =3309a(10) =12183a(11) =47133a(12) =190061a(13) =796405a(14) =3456405a(15) =15501183a(16) =71681170a(17) =341209173a(18) =1669411182a(19) =8384579797a(20) =43180474608a(21) =227797465130