E.g.f.: 1 - (1-x)*(tan(x) + sec(x)).

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =3a(5) =9a(6) =35a(7) =155a(8) =791a(9) =4529a(10) =28839a(11) =201939a(12) =1542739a(13) =12767689a(14) =113794603a(15) =1086657403a(16) =11068604847a(17) =119790363489