a(n) = binomial(n,3) - binomial(floor(n/2),3) - binomial(ceiling(n/2),3).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =4a(5) =9a(6) =18a(7) =30a(8) =48a(9) =70a(10) =100a(11) =135a(12) =180a(13) =231a(14) =294a(15) =364a(16) =448a(17) =540a(18) =648a(19) =765a(20) =900a(21) =1045a(22) =1210a(23) =1386a(24) =1584a(25) =1794a(26) =2028a(27) =2275a(28) =2548a(29) =2835