a(n) = (5*2^(n+2) - 3*n*2^n - 2*(-1)^n) / 18.

Terms

a(0) =1a(1) =2a(2) =3a(3) =5a(4) =7a(5) =9a(6) =7a(7) =-7a(8) =-57a(9) =-199a(10) =-569a(11) =-1479a(12) =-3641a(13) =-8647a(14) =-20025a(15) =-45511a(16) =-101945a(17) =-225735a(18) =-495161a(19) =-1077703a(20) =-2330169a(21) =-5009863a(22) =-10718777a(23) =-22835655a(24) =-48467513a(25) =-102527431a(26) =-216239673a(27) =-454848967a(28) =-954437177a(29) =-1998352839