Not of the form x^2 + x*y + y^2.

Terms

a(0) =2a(1) =5a(2) =6a(3) =8a(4) =10a(5) =11a(6) =14a(7) =15a(8) =17a(9) =18a(10) =20a(11) =22a(12) =23a(13) =24a(14) =26a(15) =29a(16) =30a(17) =32a(18) =33a(19) =34a(20) =35a(21) =38a(22) =40a(23) =41a(24) =42a(25) =44a(26) =45a(27) =46a(28) =47a(29) =50