Number of n-node connected graphs with one cycle, possibly of length 1 or 2.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =20a(5) =49a(6) =118a(7) =300a(8) =765a(9) =1998a(10) =5255a(11) =14027a(12) =37670a(13) =102095a(14) =278262a(15) =763022a(16) =2101905a(17) =5816142a(18) =16153148a(19) =45017423a(20) =125836711a(21) =352723949a(22) =991143727a(23) =2791422887a(24) =7877935985a(25) =22275473767a(26) =63096075118a(27) =179012076933