Number of polygons on n vertices with exactly three faces.

Terms

a(0) =0a(1) =4a(2) =16a(3) =60a(4) =224a(5) =840a(6) =3168a(7) =12012a(8) =45760a(9) =175032a(10) =671840a(11) =2586584a(12) =9984576a(13) =38630800a(14) =149768640a(15) =581690700a(16) =2262890880a(17) =8815845720a(18) =34389986400a(19) =134312002440a(20) =525129633600