a(n) = n*(n+1)*(7*n+2)/6.

Terms

a(0) =0a(1) =3a(2) =16a(3) =46a(4) =100a(5) =185a(6) =308a(7) =476a(8) =696a(9) =975a(10) =1320a(11) =1738a(12) =2236a(13) =2821a(14) =3500a(15) =4280a(16) =5168a(17) =6171a(18) =7296a(19) =8550a(20) =9940a(21) =11473a(22) =13156a(23) =14996a(24) =17000a(25) =19175a(26) =21528a(27) =24066a(28) =26796a(29) =29725