Sum of diagonal elements and those below it for a square matrix of integers, starting with 1.

Terms

a(0) =1a(1) =8a(2) =34a(3) =100a(4) =235a(5) =476a(6) =868a(7) =1464a(8) =2325a(9) =3520a(10) =5126a(11) =7228a(12) =9919a(13) =13300a(14) =17480a(15) =22576a(16) =28713a(17) =36024a(18) =44650a(19) =54740a(20) =66451a(21) =79948a(22) =95404a(23) =113000a(24) =132925a(25) =155376a(26) =180558a(27) =208684a(28) =239975a(29) =274660