Number of connected loopless multigraphs with n edges.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =12a(5) =33a(6) =103a(7) =333a(8) =1183a(9) =4442a(10) =17576a(11) =72810a(12) =314595a(13) =1410139a(14) =6541959a(15) =31322474a(16) =154468852a(17) =783240943a(18) =4077445511a(19) =21765312779a(20) =118999764062a(21) =665739100725