Expansion of g^2/(1 - x^2*g), where g = 1+x*g^2 is the g.f. of A000108.

Terms

a(0) =1a(1) =2a(2) =6a(3) =17a(4) =52a(5) =164a(6) =534a(7) =1780a(8) =6049a(9) =20881a(10) =73025a(11) =258192a(12) =921425a(13) =3314828a(14) =12008581a(15) =43770386a(16) =160405503a(17) =590677952a(18) =2184525966a(19) =8110602183a(20) =30218928061a(21) =112952845315a(22) =423435589762