Number of integral points in a certain sequence of open quadrilaterals.

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =6a(5) =9a(6) =13a(7) =18a(8) =24a(9) =31a(10) =39a(11) =47a(12) =56a(13) =66a(14) =77a(15) =89a(16) =102a(17) =115a(18) =129a(19) =144a(20) =160a(21) =177a(22) =195a(23) =213a(24) =232a(25) =252a(26) =273a(27) =295a(28) =318a(29) =341