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

Terms

a(0) =0a(1) =30a(2) =150a(3) =525a(4) =1568a(5) =4284a(6) =11070a(7) =27555a(8) =66792a(9) =158730a(10) =371462a(11) =858585a(12) =1964160a(13) =4454136a(14) =10024254a(15) =22410063a(16) =49803560a(17) =110096070a(18) =242215974a(19) =530573637a(20) =1157621280a(21) =2516574900a(22) =5452586750a(23) =11777596155a(24) =25367140008a(25) =54492385794a(26) =116769410310a(27) =249644959665