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

Terms

a(0) =0a(1) =1a(2) =6a(3) =22a(4) =66a(5) =151a(6) =312a(7) =564a(8) =984a(9) =1601a(10) =2478a(11) =3622a(12) =5242a(13) =7271a(14) =9856a(15) =13124a(16) =17296a(17) =22229a(18) =28286a(19) =35306a(20) =43850a(21) =53891a(22) =65520a(23) =78624a(24) =94272a(25) =111977a(26) =131990a(27) =154514a(28) =180290a(29) =208611