From paths in the plane.

Terms

a(0) =1a(1) =30a(2) =330a(3) =2145a(4) =10010a(5) =37128a(6) =116280a(7) =319770a(8) =793155a(9) =1808950a(10) =3848130a(11) =7719075a(12) =14725620a(13) =26898080a(14) =47303520a(15) =80454132a(16) =132835365a(17) =213578430a(18) =335304970a(19) =515175045a(20) =776173134a(21) =1148670600a(22) =1672307000a(23) =2398236750a(24) =3391791975a(25) =4735616886a(26) =6533333730a(27) =8913805255