A simple grammar: cycles of rooted cycles.

Terms

a(0) =0a(1) =0a(2) =2a(3) =3a(4) =20a(5) =90a(6) =714a(7) =5460a(8) =54704a(9) =580608a(10) =7214040a(11) =96932880a(12) =1452396912a(13) =23507621280a(14) =414102201408