Number of non-equivalent ways to choose two points in an equilateral triangle grid of side n.

Terms

a(0) =0a(1) =1a(2) =4a(3) =10a(4) =22a(5) =41a(6) =72a(7) =116a(8) =180a(9) =265a(10) =380a(11) =526a(12) =714a(13) =945a(14) =1232a(15) =1576a(16) =1992a(17) =2481a(18) =3060a(19) =3730a(20) =4510a(21) =5401a(22) =6424a(23) =7580a(24) =8892a(25) =10361a(26) =12012a(27) =13846a(28) =15890a(29) =18145