a(0)=0, a(1)=0; for n>1, a(n) = a(n-1) + (n-4)*a(n-2) + 1.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =3a(5) =6a(6) =13a(7) =32a(8) =85a(9) =246a(10) =757a(11) =2480a(12) =8537a(13) =30858a(14) =116229a(15) =455668a(16) =1850417a(17) =7774102a(18) =33679941a(19) =150291472a(20) =689170529a(21) =3244125554a(22) =15649195077a(23) =77287580604a(24) =390271482145