a(n) = 2 + n^2*floor((n+3)/2) + floor(3*n/2).

Terms

a(0) =5a(1) =13a(2) =33a(3) =56a(4) =109a(5) =155a(6) =257a(7) =334a(8) =501a(9) =617a(10) =865a(11) =1028a(12) =1373a(13) =1591a(14) =2049a(15) =2330a(16) =2917a(17) =3269a(18) =4001a(19) =4432a(20) =5325a(21) =5843a(22) =6913a(23) =7526a(24) =8789a(25) =9505a(26) =10977a(27) =11804a(28) =13501a(29) =14447