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

Terms

a(0) =0a(1) =0a(2) =24a(3) =720a(4) =6300a(5) =34812a(6) =135552a(7) =436944a(8) =1198968a(9) =2929656a(10) =6516984a(11) =13502448a(12) =26208516a(13) =48407988a(14) =85481280a(15) =145200888a(16) =238502808a(17) =380729160a(18) =591761304a(19) =899049096a(20) =1336994100a(21) =1950873276a(22) =2798226336a(23) =3952174032a(24) =5500597632a(25) =7555866072a(26) =10253438688