Number of loopless multigraphs on infinite set of nodes with n edges.

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =23a(5) =66a(6) =212a(7) =686a(8) =2389a(9) =8682a(10) =33160a(11) =132277a(12) =550835a(13) =2384411a(14) =10709827a(15) =49782637a(16) =238998910a(17) =1182772364a(18) =6023860266a(19) =31525780044a(20) =169316000494a(21) =932078457785