a(n) = 2^n*binomial(n,2).

Terms

a(0) =0a(1) =0a(2) =4a(3) =24a(4) =96a(5) =320a(6) =960a(7) =2688a(8) =7168a(9) =18432a(10) =46080a(11) =112640a(12) =270336a(13) =638976a(14) =1490944a(15) =3440640a(16) =7864320a(17) =17825792a(18) =40108032a(19) =89653248a(20) =199229440a(21) =440401920a(22) =968884224a(23) =2122317824a(24) =4630511616a(25) =10066329600a(26) =21810380800a(27) =47110422528