Expansion of e.g.f. sech(x)/(1-x).

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =17a(5) =85a(6) =449a(7) =3143a(8) =26529a(9) =238761a(10) =2337089a(11) =25707979a(12) =311198513a(13) =4045580669a(14) =56438768385a(15) =846581525775