a(n) = 1 + (26*n+17+7*n^2)*n/2.

Terms

a(0) =1a(1) =26a(2) =98a(3) =238a(4) =467a(5) =806a(6) =1276a(7) =1898a(8) =2693a(9) =3682a(10) =4886a(11) =6326a(12) =8023a(13) =9998a(14) =12272a(15) =14866a(16) =17801a(17) =21098a(18) =24778a(19) =28862a(20) =33371a(21) =38326a(22) =43748a(23) =49658a(24) =56077a(25) =63026a(26) =70526a(27) =78598a(28) =87263a(29) =96542