a(0) = 0, a(1) = 1, for n > 1, a(n) = 2^(n+1) - 3*(sigma((2^n)-1) - sigma((2^(n-1))-1)).

Terms

a(0) =0a(1) =1a(2) =-1a(3) =4a(4) =-16a(5) =40a(6) =-88a(7) =184a(8) =-400a(9) =544a(10) =-784a(11) =2224a(12) =-11536a(13) =18016a(14) =-10240a(15) =16384a(16) =-86560a(17) =203296a(18) =-503296a(19) =896512a(20) =-2329600a(21) =2795776a(22) =-1942528a(23) =8805088a(24) =-54906208a(25) =77129728a(26) =-30207616a(27) =70521376a(28) =-383472160a(29) =840798784