E.g.f.: -log(cos(x) - x).

Terms

a(0) =1a(1) =2a(2) =5a(3) =20a(4) =109a(5) =736a(6) =5977a(7) =56608a(8) =612729a(9) =7461376a(10) =100954061a(11) =1502526464a(12) =24395453861a(13) =429099278336