Binomial(n-1,3)+3*binomial(n-1,4)+6*binomial(n-1,5)+5*binomial(n-1,6).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =7a(5) =31a(6) =106a(7) =301a(8) =742a(9) =1638a(10) =3312a(11) =6237a(12) =11077a(13) =18733a(14) =30394a(15) =47593a(16) =72268a(17) =106828a(18) =154224a(19) =218025a(20) =302499a(21) =412699a(22) =554554a(23) =734965a(24) =961906a(25) =1244530a(26) =1593280a(27) =2020005a(28) =2538081a(29) =3162537