a(n) = n*(13*n-3)/2.

Terms

a(0) =0a(1) =5a(2) =23a(3) =54a(4) =98a(5) =155a(6) =225a(7) =308a(8) =404a(9) =513a(10) =635a(11) =770a(12) =918a(13) =1079a(14) =1253a(15) =1440a(16) =1640a(17) =1853a(18) =2079a(19) =2318a(20) =2570a(21) =2835a(22) =3113a(23) =3404a(24) =3708a(25) =4025a(26) =4355a(27) =4698a(28) =5054a(29) =5423