Number of quadrilaterals in a triangular matchstick arrangement of side n.

Terms

a(0) =0a(1) =0a(2) =6a(3) =33a(4) =102a(5) =243a(6) =492a(7) =894a(8) =1500a(9) =2370a(10) =3570a(11) =5175a(12) =7266a(13) =9933a(14) =13272a(15) =17388a(16) =22392a(17) =28404a(18) =35550a(19) =43965a(20) =53790a(21) =65175a(22) =78276a(23) =93258a(24) =110292a(25) =129558a(26) =151242a(27) =175539a(28) =202650a(29) =232785