Number of ways to choose four points in an n X n grid (or geoplane).

Terms

a(0) =0a(1) =1a(2) =126a(3) =1820a(4) =12650a(5) =58905a(6) =211876a(7) =635376a(8) =1663740a(9) =3921225a(10) =8495410a(11) =17178876a(12) =32795126a(13) =59626385a(14) =103962600a(15) =174792640a(16) =284660376a(17) =450710001a(18) =695946630a(19) =1050739900a(20) =1554599970a(21) =2258257001a(22) =3226076876a(23) =4538847600a(24) =6296972500a(25) =8624108025a(26) =11671285626