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

Terms

a(0) =-1a(1) =6a(2) =-4a(3) =0a(4) =-16a(5) =-24a(6) =-64a(7) =-120a(8) =-256a(9) =-504a(10) =-1024a(11) =-2040a(12) =-4096a(13) =-8184a(14) =-16384a(15) =-32760a(16) =-65536a(17) =-131064a(18) =-262144a(19) =-524280a(20) =-1048576a(21) =-2097144a(22) =-4194304a(23) =-8388600a(24) =-16777216a(25) =-33554424a(26) =-67108864a(27) =-134217720a(28) =-268435456a(29) =-536870904