a(n) = (5*2^n - 5*(-1)^n - 3*n*(-1)^n) / 9.

Terms

a(0) =0a(1) =2a(2) =1a(3) =6a(4) =7a(5) =20a(6) =33a(7) =74a(8) =139a(9) =288a(10) =565a(11) =1142a(12) =2271a(13) =4556a(14) =9097a(15) =18210a(16) =36403a(17) =72824a(18) =145629a(19) =291278a(20) =582535a(21) =1165092a(22) =2330161a(23) =4660346a(24) =9320667a(25) =18641360a(26) =37282693a(27) =74565414a(28) =149130799a(29) =298261628