Number of ways to place 2 points on a triangular grid of side n so that they are not adjacent.

Terms

a(0) =0a(1) =6a(2) =27a(3) =75a(4) =165a(5) =315a(6) =546a(7) =882a(8) =1350a(9) =1980a(10) =2805a(11) =3861a(12) =5187a(13) =6825a(14) =8820a(15) =11220a(16) =14076a(17) =17442a(18) =21375a(19) =25935a(20) =31185a(21) =37191a(22) =44022a(23) =51750a(24) =60450a(25) =70200a(26) =81081a(27) =93177a(28) =106575a(29) =121365