a(n) = 2^n*(n^2 - n + 4)-4.

Terms

a(0) =0a(1) =4a(2) =20a(3) =76a(4) =252a(5) =764a(6) =2172a(7) =5884a(8) =15356a(9) =38908a(10) =96252a(11) =233468a(12) =557052a(13) =1310716a(14) =3047420a(15) =7012348a(16) =15990780a(17) =36175868a(18) =81264636a(19) =181403644a(20) =402653180a(21) =889192444a(22) =1954545660a(23) =4278190076a(24) =9328132092a(25) =20266876924a(26) =43889197052