a(n) = (n-1)*(n+3) - 2^n + 4.

Terms

a(0) =0a(1) =2a(2) =5a(3) =8a(4) =9a(5) =4a(6) =-15a(7) =-64a(8) =-175a(9) =-412a(10) =-903a(11) =-1904a(12) =-3927a(13) =-7996a(14) =-16159a(15) =-32512a(16) =-65247a(17) =-130748a(18) =-261783a(19) =-523888a(20) =-1048135a(21) =-2096668a(22) =-4193775a(23) =-8388032a(24) =-16776591a(25) =-33553756a(26) =-67108135a(27) =-134216944a(28) =-268434615a(29) =-536870012