2^(n-1)*(n^2+2n+2).

Terms

a(0) =1a(1) =5a(2) =20a(3) =68a(4) =208a(5) =592a(6) =1600a(7) =4160a(8) =10496a(9) =25856a(10) =62464a(11) =148480a(12) =348160a(13) =806912a(14) =1851392a(15) =4210688a(16) =9502720a(17) =21299200a(18) =47448064a(19) =105119744a(20) =231735296a(21) =508559360a(22) =1111490560a(23) =2420113408a(24) =5251268608a(25) =11358175232a(26) =24494735360