a(n) = F(n+7) - (1/2)*(n^3+2*n^2+13*n+26) where F(i) is a Fibonacci number (A000045).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =2a(5) =11a(6) =37a(7) =98a(8) =225a(9) =470a(10) =919a(11) =1713a(12) =3082a(13) =5400a(14) =9274a(15) =15688a(16) =26236a(17) =43499a(18) =71655a(19) =117466a(20) =191875a(21) =312590a(22) =508265a(23) =825265a(24) =1338612a(25) =2169696a(26) =3514932a(27) =5692128a(28) =9215510a(29) =14917115