Triangles in open triangular matchstick arrangement (triangle minus one side) of side n.

Terms

a(0) =0a(1) =0a(2) =2a(3) =7a(4) =17a(5) =33a(6) =57a(7) =90a(8) =134a(9) =190a(10) =260a(11) =345a(12) =447a(13) =567a(14) =707a(15) =868a(16) =1052a(17) =1260a(18) =1494a(19) =1755a(20) =2045a(21) =2365a(22) =2717a(23) =3102a(24) =3522a(25) =3978a(26) =4472a(27) =5005a(28) =5579a(29) =6195