a(n) = binomial(n,2)*(2^(n-2) - n + 1).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =6a(5) =40a(6) =165a(7) =546a(8) =1596a(9) =4320a(10) =11115a(11) =27610a(12) =66858a(13) =158808a(14) =371553a(15) =858690a(16) =1964280a(17) =4454272a(18) =10024407a(19) =22410234a(20) =49803750a(21) =110096280a(22) =242216205a(23) =530573890a(24) =1157621556a(25) =2516575200a(26) =5452587075a(27) =11777596506a(28) =25367140386a(29) =54492386200