a(0)=0, a(1)=0; for n>1, a(n) = a(n-1) + (n+3)*a(n-2) + 1.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =10a(5) =27a(6) =118a(7) =389a(8) =1688a(9) =6357a(10) =28302a(11) =117301a(12) =541832a(13) =2418649a(14) =11629794a(15) =55165477a(16) =276131564a(17) =1379441105a(18) =7178203950a(19) =37525908261a(20) =202624599112