Related to Clebsch-Gordan formulas.

Terms

a(0) =1a(1) =1a(2) =3a(3) =14a(4) =80a(5) =468a(6) =2268a(7) =10224a(8) =313632a(9) =9849600a(10) =21954240a(11) =-8894136960a(12) =-105857556480