Number of 4-valent (or quartic) graphs with n nodes.

Terms

a(0) =1a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =1a(7) =2a(8) =6a(9) =16a(10) =60a(11) =266a(12) =1547a(13) =10786a(14) =88193a(15) =805579a(16) =8037796a(17) =86223660a(18) =985883873a(19) =11946592242a(20) =152808993767