a(n) = Sum_{i+j+k+l=n, i,j,k,l >= 1} sigma(i)*sigma(j)*sigma(k)*sigma(l).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =12a(5) =70a(6) =280a(7) =885a(8) =2364a(9) =5586a(10) =12000a(11) =23870a(12) =44660a(13) =79272a(14) =134768a(15) =220565a(16) =349440a(17) =538270a(18) =807840a(19) =1187004a(20) =1706840a(21) =2415150a(22) =3354120a(23) =4601870a(24) =6209612a(25) =8303610a(26) =10935960a(27) =14309640a(28) =18460260a(29) =23708184