a(n) = (n-2)*(n-3)*2^(n-2)+2^n-2.

Terms

a(0) =1a(1) =2a(2) =6a(3) =22a(4) =78a(5) =254a(6) =766a(7) =2174a(8) =5886a(9) =15358a(10) =38910a(11) =96254a(12) =233470a(13) =557054a(14) =1310718a(15) =3047422a(16) =7012350a(17) =15990782a(18) =36175870a(19) =81264638a(20) =181403646a(21) =402653182a(22) =889192446a(23) =1954545662a(24) =4278190078a(25) =9328132094a(26) =20266876926a(27) =43889197054a(28) =94757715966a(29) =204010946558