a(n) = n*(1-3n+2*n^2+2*n^3)/2.

Terms

a(0) =0a(1) =1a(2) =19a(3) =96a(4) =298a(5) =715a(6) =1461a(7) =2674a(8) =4516a(9) =7173a(10) =10855a(11) =15796a(12) =22254a(13) =30511a(14) =40873a(15) =53670a(16) =69256a(17) =88009a(18) =110331a(19) =136648a(20) =167410a(21) =203091a(22) =244189a(23) =291226a(24) =344748a(25) =405325a(26) =473551a(27) =550044a(28) =635446a(29) =730423