a(n) = C(n,7)-1.

Terms

a(0) =0a(1) =7a(2) =35a(3) =119a(4) =329a(5) =791a(6) =1715a(7) =3431a(8) =6434a(9) =11439a(10) =19447a(11) =31823a(12) =50387a(13) =77519a(14) =116279a(15) =170543a(16) =245156a(17) =346103a(18) =480699a(19) =657799a(20) =888029a(21) =1184039a(22) =1560779a(23) =2035799a(24) =2629574a(25) =3365855a(26) =4272047a(27) =5379615a(28) =6724519a(29) =8347679