a(n) = n + (n-1)*(n-2) + (n-3)*(n-4)*(n-5) + (n-6)*(n-7)*(n-8)*(n-9) + ... + ...*1.

Terms

a(0) =0a(1) =1a(2) =3a(3) =5a(4) =11a(5) =19a(6) =32a(7) =62a(8) =112a(9) =191a(10) =316a(11) =558a(12) =988a(13) =1711a(14) =2864a(15) =4661a(16) =7703a(17) =12883a(18) =21626a(19) =35989a(20) =58826a(21) =94217a(22) =150017a(23) =239047a(24) =382676a(25) =614425a(26) =984452a(27) =1565021a(28) =2458810a(29) =3826746