Number of quadrilaterals on an n X n grid (or geoboard).

Terms

a(0) =0a(1) =1a(2) =94a(3) =1758a(4) =13698a(5) =70539a(6) =266320a(7) =837060a(8) =2260984a(9) =5460657a(10) =12046294a(11) =24775158a(12) =47847538a(13) =87967579a(14) =154764912a(15) =262110940a(16) =429480216a(17) =684015621a(18) =1061192670a(19) =1609449754a(20) =2390049498a(21) =3483126407