Number of connected graphs with <= n edges.

Terms

a(0) =1a(1) =2a(2) =5a(3) =10a(4) =22a(5) =52a(6) =131a(7) =358a(8) =1068a(9) =3390a(10) =11461a(11) =40964a(12) =153786a(13) =603927a(14) =2471798a(15) =10509270a(16) =46296937a(17) =210848414a(18) =990794383a(19) =4795761825a(20) =23875074600a(21) =122086530809a(22) =640484152252