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

Terms

a(0) =1a(1) =-1a(2) =-15a(3) =-71a(4) =-255a(5) =-799a(6) =-2303a(7) =-6271a(8) =-16383a(9) =-41471a(10) =-102399a(11) =-247807a(12) =-589823a(13) =-1384447a(14) =-3211263a(15) =-7372799a(16) =-16777215a(17) =-37879807a(18) =-84934655a(19) =-189267967a(20) =-419430399a(21) =-924844031a(22) =-2030043135a(23) =-4437573631a(24) =-9663676415a(25) =-20971519999a(26) =-45365592063