Number of trapezoids, distinct up to congruence, on an n X n grid (or geoboard).

Terms

a(0) =0a(1) =1a(2) =9a(3) =43a(4) =141a(5) =343a(6) =766a(7) =1415a(8) =2517a(9) =4129a(10) =6545a(11) =9505a(12) =14230a(13) =19444a(14) =26733a(15) =36208a(16) =48029a(17) =60675a(18) =78729a(19) =96866a(20) =122433a(21) =151288a(22) =184072a(23) =217998a(24) =266775a(25) =315096a(26) =371138a(27) =435153a(28) =512549a(29) =585240