a(n) = a(n-1) + 2*(n-1)*a(n-2).

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =25a(5) =81a(6) =331a(7) =1303a(8) =5937a(9) =26785a(10) =133651a(11) =669351a(12) =3609673a(13) =19674097a(14) =113525595a(15) =664400311a(16) =4070168161a(17) =25330978113a(18) =163716695587