a(n) = binomial(2*n-1,n) - n*(n-1) - 1.

Terms

a(0) =0a(1) =0a(2) =3a(3) =22a(4) =105a(5) =431a(6) =1673a(7) =6378a(8) =24237a(9) =92287a(10) =352605a(11) =1351945a(12) =5200143a(13) =20058117a(14) =77558549a(15) =300539954a(16) =1166802837a(17) =4537567343a(18) =17672631557a(19) =68923264029a(20) =269128936799