a(n) = n*(7*n + 1)/2.

Terms

a(0) =0a(1) =4a(2) =15a(3) =33a(4) =58a(5) =90a(6) =129a(7) =175a(8) =228a(9) =288a(10) =355a(11) =429a(12) =510a(13) =598a(14) =693a(15) =795a(16) =904a(17) =1020a(18) =1143a(19) =1273a(20) =1410a(21) =1554a(22) =1705a(23) =1863a(24) =2028a(25) =2200a(26) =2379a(27) =2565a(28) =2758a(29) =2958